In the ancient Ruby, symbols are represented by integers. In that era,
to get string representation from integers, we used `Integer#id2sym`
method. Later, `Symbol` was introduced, and `id2sym` was used for
compatibility. Today, no one uses `id2sym` any longer. It is described
in ISO 30170:2012 standard but I consider it as a mistake.

Enclose the table contents of `opcode.md` as code

As CRuby behaves.

Fix typo in `mrbgems/mruby-io/mrblib/io.rb`; ref cb55e7ec

When the receiver is the instance of subclass of `String`.

- `String#each_char`
- `String#each_line`
- `String#partition`

It used to be an alias to `IO#each_byte` but those methods should have
behave differently.

Even called for subclass of `Array`, according to new Ruby3.0 behavior.
Other methods of `Array` behave as Ruby3.0 from the first hand.

Fix symbol leak in `exc_to_s()`

Fix annotations for inline iseq of `Class.new` [ci skip]

Introduce `MRB_GC_RED`

- stop cross building
- add more gems for tests
- add `bintest`

Avoid ill-advised `goto`

Following functions are defined in `mrblib/numeric.c`:

- `Integer#ceil`
- `Integer#floor`
- `Integer#round`
- `Integer#truncate`

- `Random.rand` with `MRB_NO_FLOAT` behaves as `Random.rand(100)`
- `Random.rand(i)` where `i<0` raises `ArgumentError`

Replaces the magic number `7` except in `src/gc.c`.

ref #5362

- `_raw` does not describe the nature of the function
- the function protect errors during C function execution

Remove unused struct in `include/mruby/variable.h`

Remove unused `#include` in `complex.c` and `rational.c`

Remove `MRB_API` from `mrb_get_backtrace` definition