Replace the fixed-size struct String with reference-counted std::strings

[Rangi42]

We tried this before in #885, now it's finally getting done!

Fixes #650. (No more string length limit!)

Be sure to rebase this, don't squash merge it, so the commits are preserved!

Still to do (but in their own individual PRs):

• Fix the memory leaks, re-enable ASan, and see if Find and fix RGBASM memory leaks #709 can be completed. (There are still some newly allocated strings which are never deleted.)
• Stop using c_str(), (the main remaining use case should be just passing to printf-like functions), and see if Don't terminate RGBDS strings with a NUL #505 can be completed.

If we see a performance hit from any of this, there are possible optimizations to try:

• Use std::move on large parser values whenever possible; avoid copying strings in general.
• reserve space in strings (or other STL collections) before building them up.
• Pass std::string_view, not const std::string & (and create these from C string literals or char const * pointers-to-buffers)?
• Make parser values be std::shared_ptr<std::string>s themselves, instead of self-owned std::strings?
• Use a custom smart pointer instead of std::shared_ptr, which would be reference-counted but not atomic. (Edit: apparently some implementations of std::shared_ptr, including gcc, are not always atomic!)

[Rangi42]

This does not yet fix #709; we can't re-enable ASan for RGBASM because there are still memory leaks.

At least some of them stem from these newly allocated strings which are never deleted; probably they should be std::make_shared<std::string>s as well:

src/asm/lexer.cpp:2192:  lexerState->captureBuf = new (std::nothrow) std::vector<char>();
src/asm/parser.y:521:    $$ = new (std::nothrow) MacroArgs();
src/asm/symbol.cpp:504:  std::string_view *macro = new (std::nothrow) std::string_view(body, size);

(Possibly some of the new (std::nothrow) FileStackNodes are being leaked too?)