Move tail call optimisations to compile.c.

[stedolan]

This might work, needs more testing.

[nicowilliams]

FYI, I tested this, but it doesn't actually TCO. I believe the proximate reason to be that compiled can't have been set where it's checked. I'm not sure how the compiled check would help. We need to check the nesting_level(), unless the idea is that a parent will have been compiled, so if compiled then that must mean the bounder is lexically an ancestor, but that would mean more refactoring of compile() and expand_call_arglist(). Or can we have some other indication via bound_by that the bounder is a lexical ancestor?