feat(frontends/lean/notation_cmd): modify infixl/infixr/postfix command syntax

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(kernel/type_checker): use argument position when reporting application type mismatch errors

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/notation_cmd): add 'notation' command

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