Allow omitting type parameters

[osa1]

Now that we can distinguish type constructors from variants in syntax, when a type parameter doesn't have bounds, we should allow omitting it. E.g.

fn f1_explicit[t](a: t): t = a

fn f1_implicit(a: t): t = a

fn f2_explicit[errs](f: Fn(): Result[[E1, ..errs], ()]): Result[[..errs], ()]

fn f2_implicit(f: Fn(): Result[[E1, ..errs], ()]): Result[[..errs], ()]

We may want to do this after #46.