Allow primitives/literals to unify regardless of their mutability

[kevinbarabash]

Mutable function params gets a little wonky with generic type because you could have something like:

fn foo<T>(mut x: T) { ... }

Since primitive types like string, number, etc. are immutable the type system will complain if you try to do:

let bar = 5;
foo(bar); // error, can't pass immutable argument to mutable param

This should be allowed though since primitives/literals can never be mutated (although a mutable variable whose type is a primitive can be re-assigned to another inhabitant of primitive type).