extyper: Handle global lookup in chained postfix expressions.
extyper: Allow scope => scope inside functions.

Previously,
    void f() {
		scope void* a;
        scope void* b = a;
    }
would be marked as an error. But clearly, no scope has been breached.
This made working with delegates really clunky.
ir: Add internal UnionLiteral.
semantic: Implement interfaces.