Fix incorrect prototype on declaration of mrb_protect_atexit

build(deps): bump actions/cache from v2.1.4 to v2.1.5

`state.c` makes a prototype declaration for the private
`mrb_protect_atexit` which is defined in `error.c`. `error.c` defines
this function with a void return type, but `state.c` defines the
prototype with an `int` return type.

This mismatch prevents mruby from compiling on stricter compilers like
emscripten.

Bumps [actions/cache](https://github.com/actions/cache) from v2.1.4 to v2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.4...1a9e2138d905efd099035b49d8b7a3888c653ca8)
Signed-off-by: dependabot[bot] <support@github.com>

- remove `Integer#chr` (thus `mruby-sting-ext`) dependency
- fix the behavior when `c.is_a? String`
- fix the behavior when `c > 255`

This CI could consume too much CPU time on GitLab. Maybe we should add
resource concious CI configuration on GitLab.

chore: fix spelling

Reorganize `mcall()` in `mruby-method`

According to CodeQL instruction, `HEAD^2` is no longer needed.

build(deps): bump actions/upload-artifact from v2.2.2 to v2.2.3

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from v2.2.2 to v2.2.3.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](https://github.com/actions/upload-artifact/compare/v2.2.2...ee69f02b3dfdecd58bb31b4d133da38ba6fe3700)
Signed-off-by: dependabot[bot] <support@github.com>

Updating dreamcast_shelf build configuration.

* `--cc` print compiler name
* `--ld` print linker name

Fix heading level in `doc/guides/link.md` [ci skip]

Fix typo in `doc/guides/link.md` [ci skip]

Making a proc object static for a method with static irep

The following methods will be made static.

  - `Class#new`
  - `Proc#call`
  - `Kernel#catch`

Previously, static const RProc could not be registered as a method, but this has been changed to allow it.

Use `mrb_exec_irep()`. If possible, re-entry into the VM will be suppressed.

Note that due to the effect of being a tail-call, the backtrace of `Method#call` will be lost, and it will look as if the target method was called directly.

This change fixes the problem of infinite loops when redefining methods that make block calls using `mruby-method`.

```console
% bin/mruby -e 'mm = method(:proc); define_method(:proc, ->(*a, &b) { mm.call(*a, &b) }); p proc { 1 }'
trace (most recent call last):
        [257] -e:1
        [256] -e:1:in proc
        [255] -e:1:in proc
        ...SNIP...
        [1] -e:1:in proc
-e:1:in proc: stack level too deep (SystemStackError)
```

Change the old `mrb_exec_irep()` as-is to static `mrb_exec_irep_vm()`.
Extract the VM entry part from the old `exec_irep()` in `mruby-eval/src/eval.c` and make it the core of the new `mrb_exec_irep()`.

Fix build failures with `enable_debug` and `enable_cxx_abi`

Fix the Authors link in the README