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.
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
* small fixes
* gofmt
* fix in raw tx build
* fixes after review
* balance getter interface
* moved address and signature calculation to public key
* errors handling
* PublicKey() returns PublicKey instead of bytes slice
* fixes after review
* fixes after review
* Initial draft of the neo-go wallet
* Cleanup + more test for util package
* integrated wallet into neo-cli partially
* base wallet implementation + smartcontract code.
