py, extmod: Introduce and use MP_FALLTHROUGH macro.

Newer GCC versions are able to warn about switch cases that fall
through.  This is usually a sign of a forgotten break statement, but in
the few cases where a fall through is intended we annotate it with this
macro to avoid the warning.

py/lexer.c

@@ -346,7 +346,8 @@ STATIC void parse_string_literal(mp_lexer_t *lex, bool is_raw) {
                                 vstr_add_char(&lex->vstr, '\\');
                                 break;
                             }
-                        // Otherwise fall through.
+                            // Otherwise fall through.
+                            MP_FALLTHROUGH
                         case 'x': {
                             mp_uint_t num = 0;
                             if (!get_hex(lex, (c == 'x' ? 2 : c == 'u' ? 4 : 8), &num)) {