* switch from fork (tendermint/btcd) to orig package (btcsuite/btcd); also
- remove obsolete check in test `size != -1` is always true
- WIP as the serialization still needs to be wrapped
* WIP: wrap signature & privkey, pubkey needs to be wrapped as well
* wrap pubkey too
* use "github.com/ethereum/go-ethereum/crypto/secp256k1" if cgo is
available, else use "github.com/btcsuite/btcd/btcec" and take care of
lower-S when verifying
Annoyingly, had to disable pruning when importing
github.com/ethereum/go-ethereum/ :-/
* update comment
* update comment
* emulate signature_nocgo.go for additional benchmarks:
592bf6a59c/crypto/signature_nocgo.go (L60-L76)
* use our format (r || s) in lower-s form when in the non-cgo case
* remove comment about using the C library directly
* vendor github.com/btcsuite/btcd too
* Add test for the !cgo case
* update changelog pending
Closes#3162#3163
Refs #1958, #2091, tendermint/btcd#1
