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.

In addition to making INIT nodes work, I've added Albert's DES code as
a test case.