In this commit, we add test vectors for filter and header construction and
the code to generate them. The included test vectors are for testnet with a
value of 20 for P. The code generates filters and headers for values of 1
through 32 for P using testnet blocks. Currently, to run the code,
the `Roasbeef` fork of `btcd` (at https://github.com/roasbeef/btcd) is
required to be running locally in testnet mode; this will be changed in a
future commit after the code is merged into the `btcsuite` mainline.
