feat(frontends/lean/pp): display implicit arguments when expression contains metavariables

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): improve error message for expressions containing unsolved metavariables

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(*): error messages

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/hop_match): in locally bound variable management

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