|        Previous Name         |        New Name         |
|------------------------------|-------------------------|
| MRB_ENABLE_ALL_SYMBOLS       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_SYMBOLL_ALL       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_CXX_ABI           | MRB_USE_CXX_ABI         |
| MRB_ENABLE_CXX_EXCEPTION     | MRB_USE_CXX_EXCEPTION   |
| MRB_ENABLE_DEBUG_HOOK        | MRB_USE_DEBUG_HOOK      |
| MRB_DISABLE_DIRECT_THREADING | MRB_NO_DIRECT_THREADING |
| MRB_DISABLE_STDIO            | MRB_NO_STDIO            |
| ENABLE_LINENOISE             | MRB_USE_LINENOISE       |
| ENABLE_READLINE              | MRB_USE_READLINE        |
| DISABLE_MIRB_UNDERSCORE      | MRB_NO_MIRB_UNDERSCORE  |
| DISABLE_GEMS                 | MRB_NO_GEMS             |

* `MRB_ENABLE_SYMBOLL_ALL` seems to be a typo, so it is fixed.
* `MRB_` prefix is added to those without.
* The previous names can also be used for compatibility.

From human readable (ASCII) string representation to binary dump of
IEEE754 in little endian.

- 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.

- `MRB_WITHOUT_FLOAT` => `MRB_NO_FLOAT`
- `MRB_USE_FLOAT` => `MRB_USE_FLOAT32`

The former is to use `USE_XXX` naming convention. The latter is to make
sure `float` is 32bit float and not floating point number in general.

ref #4576 and ref #4947

* Need MRBAPI functions without `MRB_DISABLE_STDIO`:
  * mrbgems/mruby-bin-debugger
  * mrbgems/mruby-bin-mirb
  * mrbgems/mruby-bin-mrbc
  * mrbgems/mruby-bin-mruby
  * mrbgems/mruby-bin-strip
* Need `stdio.h`:
  * mrbgems/mruby-io
  * mrbgems/mruby-print
* Need `snprintf()` in `stdio.h`:
  * mrbgems/mruby-pack
  * mrbgems/mruby-sprintf

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

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

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.

For example, `"".unpack("")` evaluates to `[]`.

The pack/unpack "m" directive should be treated as a length rather than
an element count.

The argument is converted to fixnum before calling.

The `MRB_ENDIAN_BIG` macro is originally used for `NaN` boxing.
We cannot assume it is defined on every big endian platform (#4190
is the case). So instead of relying on untrusted `MRB_ENDIAN_BIG`, we
use `BYTE_ORDER` macro with a fallback function to check endian in
runtime.

The ISO standard does not include implicit type conversion using
`to_int`. This implicit conversion often causes vulnerability.
There will be no more attacks like #4120.

In addition, we have added internal convenience method `__to_int` which
does type check and conversion (from floats).

The ISO standard does not include implicit type conversion using
`to_int`, `to_str` and sometimes `to_f`. For the compactness of the
mruby implementation, maybe we should remove those implicit conversion
from mruby.

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

The fix was proposed by https://hackerone.com/aerodudrizzt

The issue (and the fix) reported by https://hackerone.com/aerodudrizzt