[with patch, not ready for review] use labels-as-values for fast_callable under gcc

[cwitty]

This patch uses gcc's labels-as-values extension to (theoretically) speed up the instruction dispatch. Unfortunately, on my 32-bit x86 laptop, it's about the same speed as before the patch (maybe a little slower). I think this is due to poor instruction scheduling (that gcc can't fix with -fno-strict-aliasing), but trying to move the instructions around by hand makes the problem worse (it adds register pressure, which makes the register allocator do very bad things on 32-bit x86).

Doctests do NOT pass, and the patch is not fully documented. (I believe the patch works; the doctests that fail are looking at fast_callable internals that changed.)