Mruby3

Files under `test/t` and `mrbgem/*/test` are for tests, not for actual
execution. So symbols in those files need not to be pre-allocated. This
change slightly reduce the memory consumption.

Remove unnecessary assignment in String#upto

This reverts commit 8746a6fe4e7bda8a0fbc0eaece9314ec51a0c255.

We already have `mrb_protect()`, `mrb_ensure()` and `mrb_rescue()`
functions. If you need to handle exceptions from C functions, use those
functions above.

`mrb_exc_protect()` takes two C functions, `body` to be executed first,
and `resc` to be executed when an error happens during `body` execution.
Since `mrb_exc_protect()` should be compiled with the proper compiler,
we will not see the problem like #5088 that was caused by `setjmp()` and
`throw` mixture.

`MRB_TRY()` does not work when compiled by C compiler with `cxx_exception`
configuration. We should explicitly warn.

`MRB_TRY()` does not work when compiled by C compiler with `cxx_exception`,
due to the mixture of `setjmp()` used by `mirb.c` and `throw` used by
the core.  The original intension of e2e6554b is to protect code from
signal interruption, but the signal interruption is not well-defined in
mruby anyway.

Also fixed the size calculation of `irep` dump, that could cause memory
corruption.

To allow C++ compilation. Fix suggested by @dearblue.

Redirect `mrb_str_to_str` to `mrb_obj_as_string` via C macro.
Inspired by #5082

Those types are `uint16_t` in definition. Also we no longer need padding
for `iseq`.

The fix was proposed by @dearblue

The fix was proposed by @dearblue

The fix was proposed by @dearblue

Need to support `int / int -> int` update.

Target `host-debug` to use `host` internal target with debugging
configuration.

The PR was from @cubicdaiya.

    e.g. symbols like "foo[]=" make invalid C codes

The original PR was skipping Ruby comments as well, but caused some
issues in test suites.

- `mrb_check_intern()` to return `mrb_value`
- `mrb_intern_check()` to return `mrb_sym` [NEW]

Other new functions:

- `mrb_intern_check_cstr()`
- `mrb_intern_check_str()`

Rename new functions:

- `mrb_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert(mrb,val,type,tname,method)`
- `mrb_check_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert_check(mrb,val,type,tname,method)`

Old names are defined by macros (support `tname` drop and
`char*` => `mrb_sym` conversion).

On platforms where `sizeof(long)` is 4, casting `(long)` can lose data
or sign information.

- Implement `Rational()` in `C`.
- Use `float` to `rational` conversion function taken from:
  https://rosettacode.org/wiki/Convert_decimal_number_to_rational#C