py: Instead of having "debug on" var, have "optimization level" var.

This allows to have multiple "optimization" levels (CPython has two
(-OO removes docstrings), we can have more).

py/lexer.h

@@ -176,3 +176,5 @@ typedef enum {
 
 mp_import_stat_t mp_import_stat(const char *path);
 mp_lexer_t *mp_lexer_new_from_file(const char *filename);
+
+extern uint mp_optimise_value;