Refactoring and style cleanup with C++

[Sylvie]

• Don't use redundant struct keyword in types.
  This regex will fix the single-lines ones: perl -pe 's/\bstruct (\w+)\b(?! \{)(?=.+[;{)]$)/\1/g'; just double-check for wrapped for (...; lines.
• Use nullptr instead of NULL.
• Use () instead of (void).
• Use references instead of NONNULL pointers (C++ doesn't support [static 1])
• Use std::variant instead of union
• Replace functions with methods: foo_Bar(struct Foo *foo, ...) into Foo::bar(...)
• Get rid of ((m|c|re)alloc|strdup)
• Define constructors with default field values (and optimize some pushes to emplacement)
• Get rid of new (std::nothrow) in favor of automatic allocation with RAII
  • This will require the rgbasm parser to use variants and complete symbols like the linker script one does
  • Re-enable ASan for rgbasm if this succeeds

  src/asm/lexer.cpp:2206:  lexerState->captureBuf = new (std::nothrow) std::vector<char>();
  src/asm/symbol.cpp:503:  std::string_view *macro = new (std::nothrow) std::string_view(body, size);
  

[Eldred Habert]

We should look into updating NONNULL to use [[gnu::nonnull]] instead.

(Also, in general, replacing __attribute__((ATTR)) with [[gnu::ATTR]] instead. This would make less code platform-specific!)

[Sylvie]

Most if not all of the NONNULLs should be references, as should many other functions which take pointers assumed not to be null.

[Sylvie]

After the last few weeks of refactoring, RAII, smart pointers, etc, I'm declaring this done. There are two new (std::nothrow)s left, both involved with how the lexer's files/mmaps/EQUs/interpolations/macro args can "leak as a feature", and fixing that (and re-enabling ASan) is a matter for #709.