[MLton] speeding up type checking by caching

[Stephen Weeks]

> What happens in the case the ~/ is on a shared FS and I'm running
> multiple instances of MLton on different machines? This may not be a
> "common case", but I'd say its not exactly uncommon.

Agreed.  I see two things we could do.  We could make one of our
assumptions about the saved world be the machine on which the world
was generated, while keeping a single world per user.  This would
rebuild the cache whenever the user switches machines.  Or we could
keep a per-machine cached world.

And I guess we still have the synchronization problem with multiple
runs of MLton simultaneously on the same machine.  So, maybe the right
answer is some simple locking.