Split "label scope" into "global scope" and "local scope", tracking both, with . and .. string constants

[Rangi42]

We already have the concept of "label scope", which is the current context's global label. #775 requested a string constant to expand to that label, which #1499 implements as .. We also want to track the "local scope" and expand it as ... This will be generalizable if/when we add nested scopes (Function.foo_loop.bar_check.no_carry).