The macro MAYBE_UNUSED, prepared by ./configure, may not be defined in
some environments such as Oracle Developer Studio 12.5 on Solaris 10.

This fixes [Bug #20963]

Thanks, nobu!

`_umul128` is specific to x86_64 platform, see higher words by
`__umulh` on arm64.

It is not "in bytes" for wide char literal.

Explaining this by reference to rb_id2str() obscures a few important
details because IDs and symbols don't map to each other perfectly (you
can have a dynamic symbol without an ID!) Also, it used to take 2
redirections to get to concrete information, and I think being more
direct is friendlier.

Redirect `rb_nogvl` blocking operations to the fiber scheduler if possible
to prevent stalling the event loop.

[Feature #20876]

* Use FL_USER0 for ELTS_SHARED

This makes space in RString for two bits for chilled strings.

* Mark strings returned by `Symbol#to_s` as chilled

[Feature #20350]

`STR_CHILLED` now spans on two user flags. If one bit is set it
marks a chilled string literal, if it's the other it marks a
`Symbol#to_s` chilled string.

Since it's not possible, and doesn't make much sense to include
debug info when `--debug-frozen-string-literal` is set, we can't
include allocation source, but we can safely include the symbol
name in the warning message, making it much easier to find the source
of the issue.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

---------

Co-authored-by: Étienne Barrié <etienne.barrie@gmail.com>
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>