The logic here is that we'll have all binary encoding/decoding done via our io
package, which simplifies error handling. This functionality doesn't belong to
util, so it's moved.
This also expands BufBinWriter with Reset() method to fit the needs of core
package.
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
