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/lexerunix.c

@@ -30,6 +30,7 @@
 #if MICROPY_HELPER_LEXER_UNIX
 
 #include <stdio.h>
+#include <stdint.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <sys/stat.h>