Idea: Static assertions

[gbutler]

@repax

If the predicate of some run-time values cannot be determined at compile time then a run-time assertion would have to be generated by the compiler. However, if the predicate can be proved at compile time then the run-time check is unnecessary and is eliminated.

Yes, nice! And this could eventually lead to supporting something like this: [Pre-RFC] Another take at clarifying `unsafe` semantics - #31 by gbutler