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

- `_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

Introducing the `mrb_protect_raw()` API function

The purpose is two-fold:

 1. to be able to specify a pointer directly when user data is used

    When using `mrb_protect()`, it is necessary to allocate objects by `mrb_obj_cptr()` function when using user data.

    Adding `mrb_protect_raw()` will make it simpler to reimplement `mrbgems/mruby-error`.

 2. to correctly unwind callinfo when an exception is raised from a C function defined as a method (the main topic)

    If a method call is made directly under `mrb_protect()` and a C function is called, control is returned from `mrb_protect()` if an exception occurs there.
    In this case, callinfo is not restored, so it is out of sync.
    Moreover, returning to mruby VM (`mrb_vm_exec()` function) in this state will indicate `ci->pc` of C function which is equal to `NULL`, and subsequent `JUMP` will cause `SIGSEGV`.

    Following is an example that actually causes `SIGSEGV`:

    - `crash.c`

      ```c
      #include <mruby.h>
      #include <mruby/compile.h>
      #include <mruby/error.h>

      static mrb_value
      level1_body(mrb_state *mrb, mrb_value self)
      {
        return mrb_funcall(mrb, self, "level2", 0);
      }

      static mrb_value
      level1(mrb_state *mrb, mrb_value self)
      {
        return mrb_protect(mrb, level1_body, self, NULL);
      }

      static mrb_value
      level2(mrb_state *mrb, mrb_value self)
      {
        mrb_raise(mrb, E_RUNTIME_ERROR, "error!");
        return mrb_nil_value();
      }

      int
      main(int argc, char *argv[])
      {
        mrb_state *mrb = mrb_open();
        mrb_define_method(mrb, mrb->object_class, "level1", level1, MRB_ARGS_NONE());
        mrb_define_method(mrb, mrb->object_class, "level2", level2, MRB_ARGS_NONE());
        mrb_p(mrb, mrb_load_string(mrb, "p level1"));
        mrb_close(mrb);
        return 0;
      }
      ```

    - compile & run

      ```console
      % `bin/mruby-config --cc --cflags --ldflags` crash.c `bin/mruby-config --libs`

      % ./a.out
      zsh: segmentation fault (core dumped)  ./a.out
      ```

    After applying this patch, it will print exception object and exit normally.

The `mrb_protect()`, `mrb_ensure()` and `mrb_rescue_exceptions()` in `mrbgems/mruby-error` have been rewritten using `mrb_protect_raw()`.

- `mrb_exc_backtrace` to implement `Exception#backtrace`
- `mrb_get_backtrace` to implement `#caller`

- add comment for unpacking
- avoid saving the symbol in a local variable

Otherwise we suffer `(unknown):0:` errors.

On configurations where `sizeof(mrb_int) > sizeof(int)`.

`MRuby::Presym` no longer needs `Rake::DSL`

The GitHub Super Linter is a more robust and better supported
tool than the current GitHub Actions we are using.

Running these checks:

ERROR_ON_MISSING_EXEC_BIT: true
VALIDATE_BASH: true
VALIDATE_BASH_EXEC: true
VALIDATE_EDITORCONFIG: true
VALIDATE_MARKDOWN: true
VALIDATE_SHELL_SHFMT: true
VALIDATE_YAML: true

https://github.com/marketplace/actions/super-linter
https://github.com/github/super-linter

Added the GitHub Super Linter badge to the README.

Also updated the pre-commit framework and added
more documentation on pre-commit.

Added one more pre-commit check: check-executables-have-shebangs

Added one extra check for merge conflicts to our
GitHub Actions.

EditorConfig and Markdown linting.

Minor grammar and spelling fixes.

Update linter.yml

Fix `enable_debug_info?` in `mrbgems/mruby-proc-ext/test/proc.rb`

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.