feat: support type casts

[DavePearce]

Currently, the following is not compiled optimally:

(defpurefun ((vanishes! :@loob) x) x)
(defpurefun ((force-bin :binary :force) x) x)

(defcolumns (A :@loob) B C)
(defconstraint c1 ()
  (if (vanishes! (force-bin A))
      (vanishes! B)
      (vanishes! C)))

The issue is that :binary :force is essentially being ignored completely. Thus, the optimiser fails to realise that we can avoid the given inverse column in this example.

A challenge here, however, is that optimisation of normalisation expressions occurs at the MIR level. Thus, we need some way to communicate to this level that the user is guaranteeing a given expression is e.g. binary in this case.