Rewrite TailCalls for performance and immutability.

While logging symbols created after typer, I discovered that
TailCalls was far and away the largest creator. It turns out
this was due to a bug where thousands of labels were eagerly
created during tail call analysis, even if the method weren't
tail recursive and no label would ever be required.

This commit shaves 10% off the total number of method symbol
creations (compiling quick.lib drops from 88K to 80K.)
Merge pull request #2407 from paulp/pr/immutable-tailcalls

Rewrite TailCalls for performance and immutability.