There's obviously some overlap with the Rain typechecker here. I've tried to
cover everything in the AST that could potentially be bound into occam at some
point in the future, even if the occam parser doesn't support it yet (so
this'll do checks for Concat and mobile allocation, for example).
The existing constant-folding code in the parser is still there, since it needs
to know whether things are constant, and A.Dimension expects an Int.
However, this pass is useful because it does a better job of constant folding
than the parser would on its own: it can fold subexpressions of expressions
that are as a whole not constant.
