Update Rust highlighting.

[eddyb]

• NUM_SUFFIX doesn't allow f(8|16|128|size) anymore
• removed int, uint and float (they haven't been in Rust for a long time)
• added u16 (it was just missing)
• grouped together all builtin types, moved all types from keywords
  • e.g. i32, str, char etc. can be used as variable names, definitely not keywords
• split "prelude" into "functions", "types" and "traits"
• included union in the same keyword name ... { rule used for trait, enum and struct

[isagalaev]

Thanks for the excellent update!