This updates the no-cgo implementations in the crypto package to use
the github.com/btcsuite/btcd/btcec/v2 module instead of the older btcec
package that was part of the main github.com/btcsuite/btcd module.

name                   old time/op  new time/op  delta
EcrecoverSignature-32   198µs ± 0%   144µs ± 0%  -27.11%
VerifySignature-32      177µs ± 0%   128µs ± 0%  -27.44%
DecompressPubkey-32    20.9µs ± 0%  10.1µs ± 0%  -51.51%

Use (*ModNScalar).IsOverHalfOrder instead of math/big.Int when checking
for malleable signatures.