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
It's the same implementation that we have in pkg/crypto (based on
https://github.com/vsergeev/btckeygenie) but with tests preserved. I don't see
any reason to port tests from it because even the pkg/crypto copy should go
away to fix#245.
With associated test and drop duplicating Uint160 implementation from
_pkg.dev. It doesn't seem to be used in pkg code at the moment, but still it
can be useful. Refs #307.
The idea here is to preserve the history of `dev` branch development and its
code when merging with the `master`. Later this code could be moved into the
masters code where appropriate.
