Refine `mrb_alloca()`

* The allocated memory is guaranteed to be aligned for any data type (it was
  not guaranteed when string type is embed).
* Make allocation size exactly specified size (does not allocate space for a
  null byte).

Silence Clang warning with `MRB_INT64` and `MRB_32BIT` in `time.c`

Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:871:15: warning: result of comparison of constant 9223372036854775807 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->sec > MRB_INT_MAX || tm->sec < MRB_INT_MIN) {
        ~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:871:40: warning: result of comparison of constant -9223372036854775808 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->sec > MRB_INT_MAX || tm->sec < MRB_INT_MIN) {
                                 ~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:887:16: warning: result of comparison of constant 9223372036854775807 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->usec > MRB_INT_MAX || tm->usec < MRB_INT_MIN) {
        ~~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:887:42: warning: result of comparison of constant -9223372036854775808 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->usec > MRB_INT_MAX || tm->usec < MRB_INT_MIN) {
                                ~~~~~~~~ ^ ~~~~~~~~~~~
  ```

Silence Clang warning with `MRB_INT32` and `MRB_64BIT` in `time.c`

Silence the following warning:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:258:60: warning: result of comparison of constant -9223372036854775808 with expression of type 'mrb_int' (aka 'int') is always false [-Wtautological-constant-out-of-range-compare]
        if ((mrb_time_int)i > MRB_TIME_MAX || MRB_TIME_MIN > i) {
                                              ~~~~~~~~~~~~ ^ ~
  ```

Merge pull request #4848 from shuujii/fix-MRB_FIXNUM_SHIFT-with-MRB_WORD_BOXING-MRB_INT32-and-MRB_64BIT

Fix `MRB_FIXNUM_SHIFT` with `MRB_WORD_BOXING`, `MRB_INT32` and `MRB_64BIT`

### Example

  ```ruby
  # example.rb
  max32 = 2**30 - 1 + 2**30
  min32 = -max32-1
  [max32, max32+1, min32, min32-1].each{|n| p [n, n.class]}
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb
  [2147483647, Float]
  [2147483648, Float]
  [-2147483648, Float]
  [-2147483649, Float]
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb
  [2147483647, Fixnum]
  [2147483648, Float]
  [-2147483648, Fixnum]
  [-2147483649, Float]
  ```

Quit `mruby -v` immediately if no program is given for Ruby compatibility

Fix `mruby --verbose` (regression by #4827)

#### Before this patch:

  ```
  $ bin/mruby --verbose -e 'p 1'
  bin/mruby: Cannot open program file: --verbose
  ```

#### After this patch:

  ```
  $ bin/mruby --verbose -e 'p 1'
  00001 NODE_SCOPE:
  (snip)
  irep 0x7fe97041df30 nregs=4 nlocals=1 pools=0 syms=1 reps=0 iseq=11
  file: -e
      1 000 OP_LOADSELF R1
  (snip)

  1
  ```

Change the URL of `mruby.org` [ci skip]

- `http` -> `https`
- Remove `www.`

Support `--` (end of options) to `mruby` command

#### Before this patch:

  ```
  $ bin/mruby -e 'p ARGV' -- -x
  bin/mruby: invalid option -- (-h will show valid options)
  ```

#### After this patch:

  ```
  $ bin/mruby -e 'p ARGV' -- -x
  ["-x"]
  ```

Add assertion to `RVALUE` size

Rename `BITSIZE` to `BIT` and `BIT` to `BIT_POS` for consistency

Fix `ARGV` value in `mruby` command (regression by #4827)

#### Before this patch:

  ```
  $ bin/mruby -e 'p ARGV' a b
  ["bin/mruby", "-e", "p ARGV", "a", "b"]
  ```

#### After this patch:

  ```
  $ bin/mruby -e 'p ARGV' a b
  ["a", "b"]
  ```

Support short options concatenation to `mruby` command

Remove unused methods of `MRubyIOTestUtil`

Use proper `PEEK` macro for `OP_EPUSH` in `patch_irep`; fix #4833

The bit width terminology is unified to `BIT` according to `MRB_INT_BIT`
and `CHAR_BIT`. Also the bit position terminology is unified to `BIT_POS`.

Allow `true`/`false` argument to `Kernel#exit`

Introduce `mrb_ssize` type for buffer size on memory; ref #4483

Previously, `mrb_int` was used as the type that represents the buffer size
on memory, but the sizes of `RString` and `RArray` exceed 6 words when
`MRB_INT64` is enabled on 32-bit CPU.

I don't think it is necessary to be able to represent the buffer size on
memory that exceeds the virtual address space. Therefore, for this purpose,
introduce `mrb_ssize` which doesn't exceed the sizes of `mrb_int` and
pointer.

I think all `mrb_int` used for this purpose should be changed to
`mrb_ssize`, but currently only the members of the structures (`RString`,
`mrb_shared_string`, `RArray` and `mrb_shared_array`) are changed.

Always enable the rational and complex literals

Release `mruby 2.1.0`.

I think they can always be enabled because the regular expression literal is
always enabled.

Refactor `mrb_string_value_cstr`

- Keep `MRB_STR_ASCII` flag.
- Avoid a string object creation.

Fix argument specs to `Kernel`