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
* Fix#130: Wrong answer for RPC server method getaccountstate
- fixed RPC Server response
- fixed RPC Server tests
- remove unused package from go.mod
* add index and type checker
* fix review comments (thx @aprasolova)
* Implemented rcp method GetAccountState
* code clean up
* Removed empty line
* Used consistently github.com/pkg/errors package. Amended error message
* Get rid of fmt.Sprintf and use either errors.Errorf or errors.Wrapf
* cosmetic changes
