Support INIT operations

These nodes are a bit tricky: they don't actually get emited during
computation, just during initialization.  Also, the Flo file doesn't
specify a destination node name for init operations.

What I do is:
* Produce a unique, virtual output node
* Fake some width inference on the constant-valued inputs to the node
* Emit an otherwise-normal looking operation

This is a bit unfortunate as we're going to end up with special cases
in all user code to handle memory initialization.  I think what I
should really be doing is attaching INIT nodes to the memories that
they initialize, but that doesn't fit in so well with the current tool
flow.