While profiling msgpack-ruby I noticed a very substantial amout of time
spent in `rb_enc_associate_index`, called by `rb_utf8_str_new`.

On that benchmark, `rb_utf8_str_new` is 33% of the total runtime,
in big part because it cause GC to trigger often, but even then
`5.3%` of the total runtime is spent in `rb_enc_associate_index`
called by `rb_utf8_str_new`.

After closer inspection, it appears that it's performing a lot of
safety check we can assert we don't need, and other extra useless
operations, because strings are first created and filled as ASCII-8BIT
and then later reassociated to the desired encoding.

By directly allocating the string with the right encoding, it allow
to skip a lot of duplicated and useless operations.

After this change, the time spent in `rb_utf8_str_new` is down
to `28.4%` of total runtime, and most of that is GC.

[Feature #18576]

Since outright renaming `ASCII-8BIT` is deemed to backward incompatible,
the next best thing would be to only change its `#inspect`, particularly
in exception messages.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>

* And simplify callers of get_actual_encoding().
* See [Feature #18949].
* See https://github.com/ruby/ruby/pull/6322#issuecomment-1242758474

These contents are purely implementation details, not worth appearing in
CAPI documents. [ci skip]

If called with an autoloaded encoding that was not yet
initialized, `rb_enc_interned_str` would crash with
a NULL pointer exception.

See: https://github.com/ruby/ruby/pull/4119#issuecomment-800189841

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

Split ruby.h