There was a check on evaluating lets which was in Blodwen but I hadn't
added to the normaliser yet! Also, normalisation needs to reduce as
patterns for unification, but not when reducing finished LHS and
argument terms. This is a bit of a hack (but then, so is the
implementation of as patterns in general...).
So, when we're checking a nested expression, we have the as pattern as a
let bound variable (so that it has the necessary computational force)
but when we compile we just pass it as an ordinary argument, then it
gets the desired behaviour in case trees.
It's not quite there yet, though, because the treatment of 'as' patterns
isn't quite right and the slightly hacky approach we're taking might not
be the best. Rethinking now...
