Thread and memory semantics are being overhauled to use lazy symbolic values,
making the system similar to the one described in the Nienhuis paper.

Everything up to uvmThreadSemantics compiles; uvmScheduler is still incomplete.
Tests currently don't work.

Changes include:

- The "symbolic" monad defines lazy computations.

- Thread semantics execute an entire block at a time.

- Thread semantics is greatly simplified, with fewer record types, queues, and
  edge cases.

- Thread semantics does not know anything about the actual values of its
  registers. Branching instructions must receive a "guess" value from the
  scheduler; this value may be a real register value or a branch prediction.

- There are now only two scheduler step types: Commit (commits a memory message)
  and Proceed (moves a thread to the next block, optionally with a branch
  prediction).

- Pluggable memory models are gone; concurrency semantics will be handled by an
  extra layer on top of the scheduler.

- DefineUtils has some shortcuts for defining monads and num wrapper types.