Packing here does not need the wildcard
Big-endian encoding to match golang Fernet
Use a 'binary' (rather than hex) HMAC
Endian-aware unpacking to support <= 1.9.2

Ideally, this should be shimmed out so that 1.9.3+ can use the
standard pack/unpack('Q>'), and it needs to be moved to a central
place where the generator and verifier can share pack, but it
illustrates an approach that could work.
Move packing/unpacking shims to a module and describe issue
Specs for packing/unpacking numbers
Fix merge conflicts