fix(kernel/typechecker): allow elaborator to infer (Type U+1)

In the new test elab8.lean, the parameter B is in (Type U+1).
Before, this commit, the type checker was forcing all metavariables that must be types to be <= (Type U). This restriction was preventing the elaborator from succeeding in reasonable cases.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/elaborator): use equality constraints instead of convertability constraints on definitions

Convertability constraints are harder to solve than equality constraints, and it seems they don't buy us anything definitions. They are just increasing the search space for the elaborator.

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