Defs in context may now have an optional type (when unknown).

During type-checking, LetIn are pushed in the context as Defs with a known
type. When a Rel pointing to a Def in the context is found, the already
computed type (if present) is used instead of re-typing the body of the
LetIn. As a result, we get a (possibly exponential) decrease in the complexity
of the typing algorithm.