This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
We copy the new serialization togic to a new package 'current' and
export the methods we will need from the migration. This ensures that we
have isolation between the legacy and current serialization methods,
cleanly inidcating what type we are using during the migration.
