py/map: Add an optional cache of (map+index) to speed up map lookups.

The existing inline bytecode caching optimisation, selected by
MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE, reserves an extra byte in the
bytecode after certain opcodes, which at runtime stores a map index of the
likely location of this field when looking up the qstr.  This scheme is
incompatible with bytecode-in-ROM, and doesn't work with native generated
code.  It also stores bytecode in .mpy files which is of a different format
to when the feature is disabled, making generation of .mpy files more
complex.

This commit provides an alternative optimisation via an approach that adds
a global cache for map offsets, then all mp_map_lookup operations use it.
It's less precise than bytecode caching, but allows the cache to be
independent and external to the bytecode that is executing.  It also works
for the native emitter and adds a similar performance boost on top of the
gain already provided by the native emitter.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/mpstate.h

@@ -231,6 +231,11 @@ typedef struct _mp_state_vm_t {
     // This is a global mutex used to make the VM/runtime thread-safe.
     mp_thread_mutex_t gil_mutex;
     #endif
+
+    #if MICROPY_OPT_MAP_LOOKUP_CACHE
+    // See mp_map_lookup.
+    uint8_t map_lookup_cache[MICROPY_OPT_MAP_LOOKUP_CACHE_SIZE];
+    #endif
 } mp_state_vm_t;
 
 // This structure holds state that is specific to a given thread.