Python bindings: expose IndexedValue

Expose edsc::IndexedValue using a syntax smilar to that of edsc::Indexed to
ensure backwards-compatibility.  It remains possible to write array-indexed
loads and stores as

    C.store([i, j], A.load([i, k]) * B.load([k, j]))

after taking a "view" of some value handle using IndexedValue as

    A = IndexedValue(originalValueHandle)

provided that all indices are also value handles.

PiperOrigin-RevId: 238421544