Implement basic JIT

For example, this is the output for *Hashtable::put*: https://gist.github.com/marco-c/9920ca772b8421d8330d
This *case* block is a fairly lengthy chunk of code. I wonder if it'd be worth turning it into its own function, to make it easier to read and reason about, and also so it shows up separately in profiles (although its own code doesn't look expensive; and any callees, like *VM.compile*, should already show up separately).