There is a security issue found in github.com/btcsuite/btcd that we don't care
about (we're only using 256k1 implementation), but GitHub complains about
it. We could update to github.com/btcsuite/btcd/btcec/v2, but it's now just a
thin wrapper over github.com/decred/dcrd/dcrec/secp256k1/v4, so we better use
it directly.
PublicKey() for PrivateKey now just can't fail and it makes no sense to return
an error from it. There is a lot of associated functionality for which this
also is true, so adjust it accordingly and simplify a lot of code.
And drop associated _pkg.dev remnants (refs. #307).
Original `dev` branch had two separate packages for public and private keys,
but those are so intertwined (`TestHelper` subpackage is a proof) that it's
better unite them and all associated code (like WIF and NEP-2) in one
package. This patch also:
* creates internal `keytestcases` package to share things with wallet (maybe
it'll be changed in some future)
* ports some tests from `dev`
* ports Verify() method for public key from `dev`
* expands TestPrivateKey() with public key check
