These instructions were not making any assignments; instead, they
were calling through to witness_message_piece_field().
This PR also renames the witness_message_piece_field() instruction
to witness_message_piece().
This defines a Sinsemilla message in terms of pieces and subpieces.
This is useful when decomposing field elements and packing them
into K-bit messages.
SinsemillaInstructions has two const generic parameters: K, which
is the number of bits in each word of the hash, and MAX_WORDS,
which is the maximum number of words the hash can process.
For Orchard, K = 10, MAX_WORDS = 253.