Add "where" clauses

[girving]

These are just like Haskell except for evaluation order. In particular, in "e1 where v = e2", e2 is evaluated before e1. If multiple variables are declared in the where clause, the dependencies between the variables must at least be acyclic. We'll have to decide whether we want to require them to be purely backwards or not (which is why this has a semantics label as well).