Because `_pp` is originally defined in `lib/mruby/core_ext.rb`, other global
functions are moved to the file.

Improved `Object#define_singleton_method`

Rename `MRB_{ENABLE,DISABLE}_` to `MRB_{USE,NO}_`; close #5163

Integrate the implementation with `Module#define_method`.

- Introduce the internal function `mrb_mod_define_method_m()` (no static)
- The `Object#define_singleton_method` method can now accept a second argument

|        Previous Name         |        New Name         |
|------------------------------|-------------------------|
| MRB_ENABLE_ALL_SYMBOLS       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_SYMBOLL_ALL       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_CXX_ABI           | MRB_USE_CXX_ABI         |
| MRB_ENABLE_CXX_EXCEPTION     | MRB_USE_CXX_EXCEPTION   |
| MRB_ENABLE_DEBUG_HOOK        | MRB_USE_DEBUG_HOOK      |
| MRB_DISABLE_DIRECT_THREADING | MRB_NO_DIRECT_THREADING |
| MRB_DISABLE_STDIO            | MRB_NO_STDIO            |
| ENABLE_LINENOISE             | MRB_USE_LINENOISE       |
| ENABLE_READLINE              | MRB_USE_READLINE        |
| DISABLE_MIRB_UNDERSCORE      | MRB_NO_MIRB_UNDERSCORE  |
| DISABLE_GEMS                 | MRB_NO_GEMS             |

* `MRB_ENABLE_SYMBOLL_ALL` seems to be a typo, so it is fixed.
* `MRB_` prefix is added to those without.
* The previous names can also be used for compatibility.

Allow to mixed and specify `*.rb` and `*.mrb` in `bin/mruby`

It is not decides by the extension.
In order to be recognized as a `.mrb` file, the following three points must be satisfied:
- File starts with "RITE"
- At least `sizeof(struct rite_binary_header)` bytes can be read
- `NUL` is included in the first 64 bytes of the file
If these are not met, it is judged as a text file and it is processed as a Ruby script.

The `bin/mruby -b` switch is still available which treats the given file as a `.mrb` file.

New `MRB_API` function:
- `include/mruby/compile.h` and `mrbgems/mruby-compiler/core/parse.y`
  - `mrb_load_detect_file_cxt()` (remove with `MRB_DISABLE_STDIO`)

NOTE:
- Even script files now always open in binary mode for `bin/mruby`.
  The `\r\n` is handled by the `nextc()` function already, so there is no problem even on Windows.
- The `nextc0()` function in `mrbgems/mruby-compiler/core/parse.y` can now specify a string buffer and a file pointer at the same time.
  In this case, get it from the string buffer first.

This patch includes modifies by comment of https://github.com/mruby/mruby/pull/5157.

Fix incorrect build name and simplify `build_config/boxing.rb` [ci skip]

`boxing-nan-m64` was duplicated.

Merge pull request #5164 from shuujii/automatically-enable-MRB_ENABLE_DEBUG_HOOK-if-mruby-bin-debugger-is-used

Automatically enable `MRB_ENABLE_DEBUG_HOOK` if `mruby-bin-debugger` is used

* Allow mixed declarations, especially loop variables
* Allow C++ style commends `//`

Fix documents for `mrb_ary_splice()` [ci skip]

Move some `.rake` files to `tasks` directory for consistency

Unable for the `MRB_TT_CPTR` object to have a singleton class

Fix typo "overfow" to "overflow" [ci skip]

This object is treated as an immediate value.

Boxing:

* `MRB_NO_BOXING`   (`mrb_value` packed in `struct`)
* `MRB_WORD_BOXING` (`mrb_value` packed in `struct`) default
* `MRB_NAN_BOXING`  (`mrb_value` packed in `double`)

`mrb_int` size

* `MRB_INT32`       (`int32_t` as `mrb_int`)
* `MRB_INT64`       (`int64` as `mrb_int`) conflict with `MRB_NAN_BOXING'

Architecture

* `MRB_64BIT`      (`sizeof(void*)` is 64 bits)
* `MRB_32BIT`      (`sizeof(void*)` is 32 bits)

Improve determining C++ compiler in `tasks/toolchains/gcc.rake`

* Consider CC envvar as C compiler on which to make the decision.
* Consider the case where C compiler is `ccache gcc`, etc.

Refactor `mrblib/mrblib.rake`

Refactor `src/mruby_core.rake`

Merge pull request #5152 from shuujii/move-lib-mruby-core-ext.rb-to-lib-mruby-core_ext.rb-for-consistency

Move `lib/mruby-core-ext.rb` to `lib/mruby/core_ext.rb` for consistency

Allow `bintest` even if build name is not `host`

Fix integer overflow error in mrdb test

Revert "Check if irep->reps is NULL in lv_defined_p"