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.
Public key is just a point, so use the coordinates obtained previously to
initialize the PublicKey structure without jumping through the hoops of
encoding/decoding.
As NEO uses P256 we can use standard crypto/elliptic library for almost
everything, the only exception being decompression of the Y coordinate. For
some reason the standard library only supports uncompressed format in its
Marshal()/Unmarshal() functions. elliptic.P256() is known to have
constant-time implementation, so it fixes#245 (and the decompression using
big.Int operates on public key, so nobody really cares about that part being
constant-time).
New decompress function is inspired by
https://stackoverflow.com/questions/46283760, even though the previous one
really did the same thing just in a little less obvious way.
Fixes things like:
* exported type/method/function X should have comment or be unexported
* comment on exported type/method/function X should be of the form "X ..."
(with optional leading article)
Refs. #213.
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
