py/scope: Shrink scope_t struct by 1 machine word.

On 32-bit archs this makes the scope_t struct 48 bytes in size, which fits
in 3 GC blocks (previously it used 4 GC blocks).  This will lead to some
savings when compiling scripts because there are usually quite a few scopes,
one for each function and class.

Note that qstrs will fit in 16 bits, this assumption is made in a few other
places.

py/scope.h

@@ -71,8 +71,8 @@ typedef struct _scope_t {
     struct _scope_t *parent;
     struct _scope_t *next;
     mp_parse_node_t pn;
-    qstr source_file;
-    qstr simple_name;
+    uint16_t source_file; // a qstr
+    uint16_t simple_name; // a qstr
     mp_raw_code_t *raw_code;
     uint8_t scope_flags;  // see runtime0.h
     uint8_t emit_options; // see compile.h