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.

tests/unicode/unicode_slice.py

@@ -0,0 +1,12 @@
+# Test slicing of Unicode strings
+
+s = "Привет"
+
+print(s[:])
+print(s[2:])
+print(s[:5])
+print(s[2:5])
+print(s[2:5:1])
+print(s[2:10])
+print(s[-3:10])
+print(s[-4:10])