Get rid of expansion of the argument which often contains complicated
macros, and simplify the failure message.

Similar to splat argument in Ruby, which be expanded to `__VA_ARGS__`
with a leading comma if any arguments given, otherwise empty.

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

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.

If __builtin_assume() is enables and RUBY_DEBUG=0, RUBY_ASSERT(expr)
will be compiled to __builtin_assume(expr) and compiler can assume
expr is true and apply aggressive optimizations. However we observed
doubtful behavior because of compiler optimizations, we introduce
new macro RUBY_ASSERT_NOASSUME to disable __builtin_assume().
With this macro, we can try without __builtin_assume().