feat(library/elaborator): be 'lazy' when normalizing terms in the elaborator

Unification constraints of the form

         ctx |- ?m[inst:i v] == T

         and

         ctx |- (?m a1 ... an) == T

are delayed by elaborator because the produce case-splits.
On the other hand, the step that puts terms is head-normal form is eagerly applied.
This is a bad idea for constraints like the two above. The elaborator will put T in head normal form
before executing process_meta_app and process_meta_inst. This is just wasted work, and creates
fully unfolded terms for solvers and provers.

The new test demonstrates the problem. In this test, we mark several terms as non-opaque.
Without this commit, the produced goal is a huge term.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>