Create and test a Cryptor class to abstract pyelliptic.

* Replace makePubCryptor and makePrivCryptor with Cryptor.
* Pass ad-hoc encryption through the Cryptor.
Abstract and test ECC creation.

* Introduce makeECC as a factory method to hide key processing.
* Purge pyelliptic imports.
* Purge most references to _myself; let's not be self-centered.