Cross compile mruby for SerenityOS (https://github.com/SerenityOS/serenity)

In additions:

* use bitmap for flags
* BER integer compression for length

Probably we should add conditions for FreeBSD, etc.

- mrb_packed_int_len()
- mrb_packed_int_encode()
- mrb_packed_int_decode()

Add instructions for installing doxygen on RHEL/Fedora/CentOS

Check the class with `I` specifier of `mrb_get_args()`

Previously, the `I` specifier only checked if the object was `MRB_TT_ISTRUCT`.
So it was at risk of getting pointers to different C structs if multiple classes were to use the `MRB_TT_ISTRUCT` instance.

Change this behavior and change the C argument corresponding to the `I` specifier to `(void *, struct RClass)`.

This change is not compatible with the previous mruby.
Please note that if the user uses the previous specifications, `SIGSEGV` may occur or the machine stack may be destroyed.

resolve #5527

`rewind_pc` should not be called when `s->pc` is `0` (top).

Last commit made `mrb_decode_insn()` to take `NULL` as `pc`.

Fix #5528
This reverts commit 59201b59; #5497

Recent peephole optimization made `ADDI/SUBI` destinations possibly
local variables.

`a=10; a+=1` to generate:

```
    1 000 OP_LOADI      R1      11              ; R1:a
```

instead of:

```
    1 000 OP_LOADI      R1      10              ; R1:a
    1 003 OP_MOVE       R2      R1              ; R1:a
    1 006 OP_ADDI       R2      1
    1 009 OP_MOVE       R1      R2              ; R1:a
```

build(deps): bump github/super-linter from 4.6.0 to 4.6.1

Bumps [github/super-linter](https://github.com/github/super-linter) from 4.6.0 to 4.6.1.
- [Release notes](https://github.com/github/super-linter/releases)
- [Commits](https://github.com/github/super-linter/compare/v4.6.0...v4.6.1)

---
updated-dependencies:
- dependency-name: github/super-linter
  dependency-type: direct:production
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Expose all header files of the gems for small compilation

After building mruby, if the user compiles and links to `libmruby.a`, expose the included directory of the captured gems.
This is a change to the `<build-dir> /lib/libmruby.flags.mak` file and will result in being added to `bin/mruby-config --cflags`.
This eliminates the need for the user to look up the path and add the compiler flag if the user wants to take advantage of her gems publishing features.

In the main build with `rake CONFIG=...`, there is no problem because it can only be seen from the gems that depends directly and indirectly as before.
However, when compiling independently by the user using `bin/mruby-config`, a header file name collision may occur if a unique header directory is added.

pre-commit autoupdate

https://pre-commit.com/#pre-commit-autoupdate

Fixed compilation error of "mruby-pack" with `MRB_NO_STDIO`

docs: fix case of mruby