py/objslice: Add support for indices() method on slice objects.

Instances of the slice class are passed to __getitem__() on objects when
the user indexes them with a slice.  In practice the majority of the time
(other than passing it on untouched) is to work out what the slice means in
the context of an array dimension of a particular length.  Since Python 2.3
there has been a method on the slice class, indices(), that takes a
dimension length and returns the real start, stop and step, accounting for
missing or negative values in the slice spec.  This commit implements such
a indices() method on the slice class.

It is configurable at compile-time via MICROPY_PY_BUILTINS_SLICE_INDICES,
disabled by default, enabled on unix, stm32 and esp32 ports.

This commit also adds new tests for slice indices and for slicing unicode
strings.

py/obj.h

@@ -778,8 +778,16 @@ static inline mp_map_t *mp_obj_dict_get_map(mp_obj_t dict) {
 // set
 void mp_obj_set_store(mp_obj_t self_in, mp_obj_t item);
 
+// slice indexes resolved to particular sequence
+typedef struct {
+    mp_int_t start;
+    mp_int_t stop;
+    mp_int_t step;
+} mp_bound_slice_t;
+
 // slice
 void mp_obj_slice_get(mp_obj_t self_in, mp_obj_t *start, mp_obj_t *stop, mp_obj_t *step);
+void mp_obj_slice_indices(mp_obj_t self_in, mp_int_t length, mp_bound_slice_t *result);
 
 // functions
 
@@ -836,13 +844,6 @@ const mp_obj_t *mp_obj_property_get(mp_obj_t self_in);
 
 // sequence helpers
 
-// slice indexes resolved to particular sequence
-typedef struct {
-    mp_uint_t start;
-    mp_uint_t stop;
-    mp_int_t step;
-} mp_bound_slice_t;
-
 void mp_seq_multiply(const void *items, size_t item_sz, size_t len, size_t times, void *dest);
 #if MICROPY_PY_BUILTINS_SLICE
 bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice_t *indexes);