Use ethereum's secp256k1 lib (#3234)

* 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:
https://github.com/ethereum/go-ethereum/blob/592bf6a59cac9697f0491b24e5093cb759d7e44c/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

crypto/encoding/amino/encode_test.go

@@ -25,9 +25,8 @@ func checkAminoBinary(t *testing.T, src, dst interface{}, size int) {
 		assert.Equal(t, byterSrc.Bytes(), bz, "Amino binary vs Bytes() mismatch")
 	}
 	// Make sure we have the expected length.
-	if size != -1 {
-		assert.Equal(t, size, len(bz), "Amino binary size mismatch")
-	}
+	assert.Equal(t, size, len(bz), "Amino binary size mismatch")
+
 	// Unmarshal.
 	err = cdc.UnmarshalBinaryBare(bz, dst)
 	require.Nil(t, err, "%+v", err)