Where fixnum overflow can happen.

- Integrate `Fixnum` and `Integer`
- Remove `Integral`
- `int / int -> int`
- Replace `mrb_fixnum()` to `mrb_int()`
- Replace `mrb_fixnum_value()` to `mrb_int_value()`.
- Use `mrb_integer_p()` instead of `mrb_fixnum_p()`

* The `Fixnum` constant is now an alias for the `Integer` class.
* Remove `struct mrb_state::fixnum_class` member.
  If necessary, use `struct mrb_state::integer_class` instead.

Except for support files e.g. `mruby-test/driver.c`, which are not
target of symbol collection via `rake gensym`.

- The `s` specifier is a string pointer obtained without performing `mrb_str_modify()`, so it cannot be changed.
- The `z` specifier cannot be changed because it is a string pointer obtained by `RSTRING_CSTR()` which returns `const char *`.

'stdio.h' is included in 'mruby.h' ('mrbconf.h').

However, keep 'stdio.h' used by mruby-test.

To unify the style of messages.

It is writing side by side with the original authors.

For efficiency with `MRB_WORD_BOXING` (implement type predicate macros for
all `enum mrb_vtype`).

`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`

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.

Because it always allocate new string. Replace with the followings:

- Use `RSRING_PTR` if string is guaranteed to be null-terminated.
- Use `mrb_string_value_cstr` or `mrb_get_args("z")` if return value isn't
  modified.

`mrb_raisef()` only takes `%S` specifier. If you don't have extra
arguments, use `mrb_raise()`.

If your system has sa_len but is not BSD-derived, define HAVE_SA_LEN=1
on mrbgem.rake or build_config.rb.

When we have "struct sockaddr_un *s_un", we could not assume *s_un
points to a memory region which size is at least sizeof(*s_un).
Even worse, it may be shorter than sizeof(struct sockaddr) on some
systems.

ref: https://github.com/iij/mruby-socket/issues/45

To avoid conflicts with multiple definitions of inet_ntop and inet_pton if compiled with # define _WIN32_WINNT _WIN32_WINNT_VISTA

C:/mingw-w64/x86_64-7.2.0-posix-seh-rt_v5-rev1/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/7.2.0/../../../../x86_64-w64-mingw32/lib/../lib/libWs2_32.a(dghfs00169.o):(.text+0x0): multiple definition of `inet_pton'
722C:/projects/iss/mruby/build/host/lib/libmruby.a(socket.o):C:/projects/iss/mruby/mrbgems/mruby-socket/src/socket.c:80: first defined here

define _WIN32_WINNT _WIN32_WINNT_VISTA