typechecker+codegen+tests: Accept never as a branch result

[cg-jl]

This affects matches specially but another test is added to make sure that assigning never is still reported as an error since it doesn't make sense to store something you're not getting.

EDIT: cc @alimpfard

[alimpfard]

I'm gonna copypaste what I said on discord:

imo => { panic() } makes it clear that this branch isn't yielding values, unlike => panic() which sounds like it's evaluating to the value of that call

But if this sounds like a good idea to @jntrnr & @awesomekling, I wouldn't mind having it