feat(frontends/lean/parser): propagate position information to expressions created by macro implemented in Lua

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): change show-expression binder name

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/elaborator): missing case

The elaborator was failing in the following scenario:
- Failing constraint of the form
     ctx |- ?m1 =:= ?m2
where
     ?m2 is assigned to ?m1,
     and ?m1 is unassigned.

has_metavar(?m2, ?m1) returns true, and a cycle is incorrectly reported.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/elaborator): only expand definitions that are not marked as hidden

The elaborator produces better proof terms. This is particularly important when we have to prove the remaining holes using tactics.
For example, in one of the tests, the elaborator was producing the sub-expression

 (λ x : N, if ((λ x::1 : N, if (P a x x::1) ⊥ ⊤) == (λ x : N, ⊤)) ⊥ ⊤)

After, this commit it produces

 (λ x : N, ¬ ∀ x::1 : N, ¬ P a x x::1)

The expressions above are definitionally equal, but the second is easier to work with.

Question: do we really need hidden definitions?
Perhaps, we can use only the opaque flag.

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