C++20 Support

[nicbarker]

This PR adds support for direct #include clay.h in C++20. The changes should be opaque to C and other languages.

The main set of changes required were:

• Macro around designated initializers because C99 uses (StructName) { members } and C++20 uses StructName { members }
• Macro __attribute__((__packed__)) and swap out for enum : uint8_t for 1 byte enums
• Reorder all designated initializer fields to be in declaration order, as required by C++20 standard (this was a pain)
• Swap out the C99 hack for detecting the alignment of a type for the supported alignof in C++20

[mikejsavage]

You could make this : uint8_t instead of a noop in C++

[nicbarker]

Nice idea, have put it in 👍