Since edf01d4e82d8e44ee30ec41fbcb7f802bc8b8c5d, fake string treats
NULL as an empty string.

* Unify length field for embedded and heap strings

The length field is of the same type and position in RString for both
embedded and heap allocated strings, so we can unify it.

* Remove RSTRING_EMBED_LEN

Remove !USE_RVARGC code

[Feature #19579]

The Variable Width Allocation feature was turned on by default in Ruby
3.2. Since then, we haven't received bug reports or backports to the
non-Variable Width Allocation code paths, so we assume that nobody is
using it. We also don't plan on maintaining the non-Variable Width
Allocation code, so we are going to remove it.

This function was added to a public header in [1] probably
unintentionally since it's not used anywhere, exposes implementation
details, and isn't related to the goals of that pull request.

[1]: 56cc3e99b6b9ec004255280337f6b8353f5e5b06

And re-embed any strings that can now fit inside the slot they've been
moved to