The benchmark results show that this feature has either a positive or
no impact on performance. The memory usage is also mostly unchanged,
except in hexapdf, where there is a decrease in RSS.

--------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------
bench           master (ms)  stddev (%)  RSS (MiB)  branch (ms)  stddev (%)  RSS (MiB)  branch 1st itr  master/branch
activerecord    70.8         2.2         56.0       71.7         2.2         56.0       0.99            0.99
erubi_rails     20.5         13.6        94.7       20.5         14.3        94.2       0.93            1.00
hexapdf         2541.0       0.7         212.8      2544.4       0.7         203.4      1.00            1.00
liquid-c        65.6         0.3         38.9       65.3         0.3         38.9       1.01            1.01
liquid-compile  63.7         0.3         34.6       61.1         0.2         34.6       1.04            1.04
liquid-render   163.1        0.1         37.1       163.3        0.1         37.1       1.00            1.00
mail            139.3        0.1         50.5       137.0        0.1         50.1       0.99            1.02
psych-load      2065.7       0.1         36.9       2068.2       0.1         37.3       1.00            1.00
railsbench      2034.6       0.5         103.9      2031.9       0.5         103.8      1.02            1.00
ruby-lsp        65.3         3.1         89.8       66.2         3.0         89.7       1.01            0.99
sequel          73.2         1.0         40.3       73.4         1.0         40.3       1.00            1.00
--------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------

RB_OBJ_WRITE already exists in rgengc.h, so we shouldn't redefine it in
gc.h.

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

Remove all usages of RCAST() so that the header file can be excluded
from ruby/ruby.h's dependency.

To fix build failures.

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

Split ruby.h