feat(library/unifier): improve failure report

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/elaborator): keep postponing delayed coercions until the type can be inferred

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/elaborator): keep postponing delayed coercions until the type can be inferred