py: Add option to cache map lookup results in bytecode.

This is a simple optimisation inspired by JITing technology: we cache in
the bytecode (using 1 byte) the offset of the last successful lookup in
a map. This allows us next time round to check in that location in the
hash table (mp_map_t) for the desired entry, and if it's there use that
entry straight away.  Otherwise fallback to a normal map lookup.

Works for LOAD_NAME, LOAD_GLOBAL, LOAD_ATTR and STORE_ATTR opcodes.

On a few tests it gives >90% cache hit and greatly improves speed of
code.

Disabled by default.  Enabled for unix and stmhal ports.

py/objtype.h

@@ -37,6 +37,10 @@ typedef struct _mp_obj_instance_t {
     // TODO maybe cache __getattr__ and __setattr__ for efficient lookup of them
 } mp_obj_instance_t;
 
+// these need to be exposed for MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE to work
+void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest);
+bool mp_obj_instance_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value);
+
 // these need to be exposed so mp_obj_is_callable can work correctly
 bool mp_obj_instance_is_callable(mp_obj_t self_in);
 mp_obj_t mp_obj_instance_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args);