witness_message() witnesses a full message given a bitstring.
The other two APIs, witness_message_piece_bitstring() and
witness_message_piece_field(), both witness a message piece, i.e.
part of a message that fits within a single base field element.
witness_message_piece_bitstring() takes in a bitstring, while
witness_message_piece_field() takes in a field element. In the
latter case, the number of words encoded must be specified.