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

Creative use of `@copydoc` Doxygen command and abusing its half-broken C
parser let us delete some lines of documentations, while preserving
document coverages.

C++ keyword `nullptr` represents a null pointer (note also that NULL is
an integer in C++ due to its design flaw).  Its type is `std::nullptr_t`,
defined in <cstddef> standard header.  Why not support it when the
backend implementation can take a null pointer as an argument.

It was unable for rb_define_global_function to take VALUE(*)(int argc,
const VLAUE *argv, VALUE self) -style function.  Test added.

rb_f_notimplement should be accepted for all possible arities.

Test provided for that.

The `using engine<...snip...>::define;` line already defines this
function.  We don't have to repeat.