[M3devel] field references in m3cg

[Jay K]

I was slow to realize this. Slow to realize how procs/labels/vars work.
Fields need to be small integers that index into a global array of trees, just like procs/labels/vars.
Types also should work this way -- that I do binary search for type id is not great.
I'll get to this pretty soon as part of making the gcc IR typeful.
After I get Hudson into better shape.

 - Jay