Support pathmap notation, recursive dependencies search, and so on.

Contrary to the name, `mrb_to_str` just checks type, no conversion.

`mrb_string_value_cstr` and `mrb_string_value_len`: obsolete
`mrb_string_cstr`: new function to retrieve NULL terminated C string
`RSTRING_CSTR`: wrapper macro of `mrb_string_cstr`

Add `mrb_noreturn` to `cmperr()` in `src/numeric.c`

Use new specifiers/modifiers of `mrb_vfromat()`

The binary sizes (gems are only `mruby-bin-mruby`) are reduced slightly in
my environment than before the introduction of new specifiers/modifiers
(5116789a) with this change.

  ------------+-------------------+-------------------+--------
   BINARY     | BEFORE (5116789a) |   AFTER (This PR) |  RATIO
  ------------+-------------------+-------------------+--------
   mruby      |      593416 bytes |      593208 bytes | -0.04%
   libmruby.a |      769048 bytes |      767264 bytes | -0.23%
  ------------+-------------------+-------------------+--------

BTW, I accidentally changed `tasks/toolchains/visualcpp.rake` at #4613,
so I put it back.

Use `mrb_int()` instead of `to_int()` in `mruby-numeric-ext`

Add constants for floating point number

Merge pull request #4613 from shuujii/change-second-argument-to-%l-of-mrb_vformat-to-size_t-from-mrb_int

Change second argument to `%l` of `mrb_vformat()` to `size_t` from `mrb_int`

- `size_t` is more commonly used.
- `len` argument of `mrb_str_new()` is `size_t`.

NOTE:

The test for `%l` is temporarily disabled because adding a new type to
`mrbgems/mruby-test/vformat.c` causes an error (memory error?) on Visual
Studio 2017 in AppVeyor.

Fix `mrb_vformat("%f")` with `MRB_USE_FLOAT`

Suppress compiler warnings for mruby-time; fix #4600

It potentially not work when `mrb_float` is `float` because `float` variable
in variable length arguments is promoted to `double`.

Also I fixed build with `MRB_WITHOUT_FLOAT`.

Warnings:

- If `MRB_TIME_T_UINT` is defined, the compiler issues a warning with an
  integer comparison of different signs.
- It is mentioned that the `usec` variable passed to the
  `mrb_to_time_t()` function may not be initialized

Change the `mrb_vformat` specifier `%d` for `int`

It potentially breaks, for example, in the case of `mrb_int` is 64-bit
and more smaller type is passed by `%d`. In fact, the problem could
become apparent when I used `%d` to `backtrace_location::lineno` in
`src/backtrace.c:mrb_unpack_backtrace()` on AppVeyor.

Therefore, change `%d` for `int` (not `mrb_int`) so that it can be
used mostly without casting.

Add new specifiers/modifiers to format string of `mrb_vfromat()`

Format sequence syntax:

  %[modifier]specifier

Modifiers:

  ----------+------------------------------------------------------------
  Modifier  | Meaning
  ----------+------------------------------------------------------------
      !     | Convert to string by corresponding `inspect` instead of
            | corresponding `to_s`.
  ----------+------------------------------------------------------------

Specifiers:

  ----------+----------------+--------------------------------------------
  Specifier | Argument Type  | Note
  ----------+----------------+--------------------------------------------
      c     | char           |
     d,i    | mrb_int        |
      f     | mrb_float      |
      l     | char*, mrb_int | Arguments are string and length.
      n     | mrb_sym        |
      s     | char*          | Argument is NUL terminated string.
      t     | mrb_value      | Convert to type (class) of object.
     v,S    | mrb_value      |
      C     | struct RClass* |
      T     | mrb_value      | Convert to real type (class) of object.
      Y     | mrb_value      | Same as `!v` if argument is `true`, `false`
            |                | or `nil`, otherwise same as `T`.
      %     | -              | Convert to percent sign itself (no argument
            |                | taken).
  ----------+----------------+--------------------------------------------

This change will increase the binary size, but replacing all format strings
with new specifiers/modifiers will decrease the size because it reduces
inline expansion of `mrb_obj_value()`, etc. at the caller.

Merge pull request #4607 from shuujii/fix-UTC-offset-representation-in-Time-to_s-on-some-environments

Fix UTC offset representation in `Time#to_s` on some environments; ref #4604

Use own implementation to calculate UTC offset on Visual Studio 2015 or
earlier or MinGW because `strftime("%z")` on these environments does not
conform C99.

`MRB_INT_MIN` is the only integer value that has no corresponding
positive integer value (i.e. `-MRB_INT_MIN` = `MRB_INT_MIN`).

Refine message to `skip` in nested `assert`

- I think "Info" is used only to `skip`, so change to "Skip".
- Changed the default value of `assert` and specify the argument explicitly
  at the caller of `assert` because it is unnatural "Assertion failed" is
  output even though the assertion doesn't fail.

== Example:

  def assert_foo(exp, act)
    assert do
      assert_equal exp[0], act[0]
      assert_equal exp[1], act[1]
    end
  end

  def assert_bar(exp, act)
    assert do
      skip
    end
  end

  def assert_baz(exp, act)
    assert do
      assert_equal exp, act
      assert_bar exp, act
    end
  end

  assert 'test#skip_in_nested_assert' do
    assert_baz 1, 1
  end

  === Before this patch:

    ?..
    Info: test#skip_in_nested_assert (core)
     - Assertion[1]
        Info: Assertion failed (core)
         - Assertion[1-2]
            Skip: Assertion failed (core)
      Total: 3
         OK: 2
         KO: 0
      Crash: 0
    Warning: 0
       Skip: 1

  === After this patch:

    ???
    Skip: test#skip_in_nested_assert (core)
     - Assertion[1]
        Skip: assert (core)
         - Assertion[1-2]
            Skip: assert (core)
      Total: 3
         OK: 0
         KO: 0
      Crash: 0
    Warning: 0
       Skip: 3

Fix "Warn if assertion is missing inside `assert`"; ref ff43b2b9

Fix Time#to_s encoding on Windows

strftime() on Windows returns locale encoding time zone for "%z" even
if MSDN says "%z" is "The offset from UTC in ISO 8601 format; no
characters if time zone is unknown" in MSDN:

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/strftime-wcsftime-strftime-l-wcsftime-l?view=vs-2019

So we need to convert encoding of string from strftime().