py: Shrink mp_arg_t struct by using reduced-size integer members.

qstrs ids are restricted to fit within 2 bytes already (eg in persistent
bytecode) so it's safe to use a uint16_t to store them in mp_arg_t.  And
the flags member only needs a maximum of 2 bytes so can also use uint16_t.

Savings in code size can be significant when many mp_arg_t structs are
used for argument parsing.  Eg, this patch reduces stmhal by 480 bytes.

py/runtime.h

@@ -52,8 +52,8 @@ typedef union _mp_arg_val_t {
 } mp_arg_val_t;
 
 typedef struct _mp_arg_t {
-    qstr qst;
+    uint16_t qst;
-    mp_uint_t flags;
+    uint16_t flags;
     mp_arg_val_t defval;
 } mp_arg_t;