Merge flow control back together after a branch about lineiform HOT 2 OPEN

Ok I think the correct answer is to stop being dumb and just build a CFG for functions. Doing a single pre-process pass before JIT compilation realistically isn't going to be very slow, and gives us good and useful information, like for example having a "complexity" metric to feed into #15. The basic block layout heuristic would also fall on its face if we ever tried to inline a JIT compiled function, since Cranelift is dumb and doesn't know that you should put cold code outside the instruction fetch pipeline.

from lineiform.

(If we want to be even smarter we could do the CFG pass async, and only block on it the first time we hit a branch)

from lineiform.