BIP158: add test vectors and generation code

In this PR, 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 PR after the code is merged into the `btcsuite` mainline.

Specification