C3 0.7.10: constdef Takes Shape

[lerno]

https://c3-lang.org/blog/c3-0-7-10-constdef-finally-takes-shape/

[lerno]

C3 0.7.10 introduces constdef, syntactically and semantically making a clear distinction between proper enums and "enums is collection of constants".

Other improvements in this release:

- Much improved MSVC cross compilation

- Quality-of-life fixes

- Custom LLVM builds to reduce external dependencies