py: Optimise lexer by exposing lexer type.

mp_lexer_t type is exposed, mp_token_t type is removed, and simple lexer
functions (like checking current token kind) are now inlined.

This saves 784 bytes ROM on 32-bit unix, 348 bytes on stmhal, and 460
bytes on bare-arm.  It also saves a tiny bit of RAM since mp_lexer_t
is a bit smaller.  Also will run a bit more efficiently.

py/builtinimport.c

@@ -127,7 +127,7 @@ STATIC void do_load(mp_obj_t module_obj, vstr_t *file) {
     }
 
     #if MICROPY_PY___FILE__
-    qstr source_name = mp_lexer_source_name(lex);
+    qstr source_name = lex->source_name;
     mp_store_attr(module_obj, MP_QSTR___file__, MP_OBJ_NEW_QSTR(source_name));
     #endif