feat(library/elaborator): improve support for dependent pairs in the elaborator

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean): allow user set constants defined in other namespaces as opaque

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(frontends/lean/parser): update comments

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