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.
In this commit, we modify regular filter construction slightly. Rather
than including each pushed data in the script, we instead just include
the script directly, which will eventually be hashed. The rationale for
doing this is two-fold:
* Most scripts today and in the foreseeable future will just be a
commitment.
* Including only the script itself and not the hash of the script
reduces the worst case filter size. Otherwise, an attacker could
include a bunch of 2 byte push datas and blow up the filter size for
all nodes.
