py: Put all global state together in state structures.

This patch consolidates all global variables in py/ core into one place,
in a global structure.  Root pointers are all located together to make
GC tracing easier and more efficient.

py/lexer.h

@@ -192,6 +192,4 @@ typedef enum {
 mp_import_stat_t mp_import_stat(const char *path);
 mp_lexer_t *mp_lexer_new_from_file(const char *filename);
 
-extern mp_uint_t mp_optimise_value;
-
 #endif // __MICROPY_INCLUDED_PY_LEXER_H__