Generate code for a simple counter example

This generates LLVM IR and a not quite complete VCD file (there's no
module information, but the header stuff is there) for a simple
counter example.  I think the current big problems are:

* Widths aren't handled correctly for EQ-like instructions (this will
  require a libflo fix).

* Wide nodes (wider than uint64_t) aren't handled by the C++ shim.

* Many Flo instructions don't have implementations

Aside from that I think this should be pretty much OK.