This adds a new SeedService to the gRPC server to generate quality
random seed in binary, hex, and mnemonic word list encodings, and
decode human-readable seed formats back into binary so they can be
used in other gRPC request messages.
The Go APIs have also been significantly improved. The pgpwordlist
package no longer has any notion of a checksum and only exists to
encode and decode directly to and from the PGP word list encoding. A
new walletseed package, inspired by the WalletSeed class from
Paymetheus, has been added to encode and decode seeds using the
checksummed mnemonic encoding.