2,291 lines are too much!  include/ruby/encoding.h became the biggest
header file once it had doxygen comments.  Let us split it into smaller
parts, so that we can better organise their contents.

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

This declaration was added at commit 0ee5a49dd4ffbbb285c25fa6cba88370ce9
without its implementation.  Must be a mistake.

Note also that we ended up having this exct same functionality
implemented under a name of rb_check_id().

This declaration was added at commit 0ee5a49dd4ffbbb285c25fa6cba88370ce9
without its implementation.  Must be a mistake.

Fixes [Feature #13381]

To fix build failures.

Devs do not love "3".  The only exception is RUBY3_KEYWORDS in parse.y,
which seems unrelated to our interests.

This shall fix compile errors.

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