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

Use of TOKEN_PASTE was a bad idea at the first place.  Just use ##
everywhere.  Nobody practically lacks token pasting.

iff means if and only if, but readers without that knowledge might
assume this to be a spelling mistake. To me, this seems like
exclusionary language that is unnecessary. Simply using "if and only if"
instead should suffice.

- When NDEBUG is defined that shall be honoured.
- When RUBY_DEBUG is defined that shall be honoured.
- When both are defined and they conflict, warnings shall be rendered.
- When nothing is specified, nothing shall happen.

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`.

To fix build failures.

Devs do not love "impl".