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
* Fixed small incosistencies related to comments and variable name
* For testing purposes we have to move the method newBlockchain from the cli/server/server.go to pkg/core/blockchain.go. In addition we have to rename it to NewBlockchainLevelDB in order to be able to export it and avoid naming conflicts. In future we still need to think how to switch between different blockchain implementation easily but for the time being this is not possible.
* Added unit tests for the rpc server
* Added unit_testnet chain fixture
* fixed port number
* Added errors handling
* move unit_testnet chain from 'cli/chains' to 'pkg/rpc/chains'
