`toregexp` is fairly similar to `concatstrings`, so this commit extracts
a helper for pushing and popping operands on the native stack.

There's probably opportunity to move some of this into lir (e.g. Alan
suggested a push_many that could use STP on ARM to push 2 at a time),
but I might save that for another day.

In gsub is used with a string replacement or a map that doesn't
have a default proc, we know for sure no code can cause the MatchData
to escape the `gsub` call.

In such case, we still have to allocate a new MatchData because we
don't know what is the lifetime of the backref, but for any subsequent
match we can re-use the MatchData we allocated ourselves, reducing
allocations significantly.

This partially fixes [Misc #20652], except when a block is used,
and partially reduce the performance impact of
abc0304cb28cb9dcc3476993bc487884c139fd11 / [Bug #17507]

```
compare-ruby: ruby 3.5.0dev (2025-02-24T09:44:57Z master 5cf146399f) +PRISM [arm64-darwin24]
built-ruby: ruby 3.5.0dev (2025-02-24T10:58:27Z gsub-elude-match da966636e9) +PRISM [arm64-darwin24]
warming up....

|                 |compare-ruby|built-ruby|
|:----------------|-----------:|---------:|
|escape           |      3.577k|    3.697k|
|                 |           -|     1.03x|
|escape_bin       |      5.869k|    6.743k|
|                 |           -|     1.15x|
|escape_utf8      |      3.448k|    3.738k|
|                 |           -|     1.08x|
|escape_utf8_bin  |      6.361k|    7.267k|
|                 |           -|     1.14x|
```

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

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

Following non-special_const literals:
* T_REGEXP

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