Refinement#import_methods imports methods from modules.
Unlike Module#include, it copies methods and adds them into the refinement,
so the refinement is activated in the imported methods.

[Bug #17429] [ruby-core:101639]

Must not be a bad idea to improve documents. [ci skip]

Truly editorial fix for comments.  This works better with Emacs'
set-justification-full function. [ci skip]

    [0] => [0, *, a]
    #=> [0] length mismatch (given 1, expected 2+) (NoMatchingPatternError)

Ignore test failures of typeprof caused by this change for now.

Deleted well over a decade ago in commit 977d66ec993b0f1892fb3e4cd8ac.

* Use the wrapper of rb_cObject instead of data access
* Replaced rest of extentions
* Updated the version guard for Data
* Added the version guard of rb_cData

Has been deprecated since 684bdf6171b76f5bc5e4f05926a5ab01ec2b4fd5.

Matz says in [ruby-core:83954] that Data should be an alias of Object.
Because rb_cData has not been deprecated, let us deprecate the constant
to make it a C-level synonym of rb_cObject.

Assertions in header files slows down an interpreter, so they should be
turned off by default (simple `make`). To enable them, define a macro
`RUBY_DEBUG=1` (e.g. `make cppflags=-DRUBY_DEBUG` or use `#define` at
the very beggining of the file. Note that even if `NDEBUG=1` is defined,
`RUBY_DEBUG=1` enables all assertions.
[Feature #16837]
related: https://github.com/ruby/ruby/pull/3120

`assert()` lines in MRI *.c is not disabled even if `RUBY_DEBUG=0` and
it can be disabled with `NDEBUG=1`. So please consider to use
`RUBY_ASSERT()` if you want to disable them when `RUBY_DEBUG=0`.

Argument of RUBY_ASSERT_FAIL is already stringized message, so no
more extra stringization should be applied.

To fix build failures.