add new BytesNoWitness method to Block
add new BytesNoWitness method to Block

This commit adds a new method to btcutil.Block for obtaining the
serialization of a Block *without* any witness data (if any).

dajohi/btcutil,btcsuite/btcutil,btcsuite/btcutil,Roasbeef/btcutil,Roasbeef/btcutil,jrick/btcutil,ppcsuite/ppcutil,dajohi/btcutil,jrick/btcutil,ppcsuite/ppcutil