Add BIP98: Fast Merkle Trees

This contains the revised draft proposal for BIP98: Fast Merkle Trees, which incorporates all the feedback received so far from the developer mailing list and in-person review. This BIP draft is considered complete by its authors.

For context, see the following mailing list discussions:

https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-September/014932.html https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-September/014935.html https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-September/015028.html https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-October/015209.html