api cleanup

[pipermerriam]

What was wrong?

• The KeyAPI object was awkward for the common use case, requiring you instantiate it everytime you wanted to do pk operations.
• The Signature.recover_* api names were misleading.
• The PrivateKey.sign api name wasn't as explicit as I would like.
• The PublicKey.recover_ api names were misleading.

How was it fixed?

• There is now a KeyAPI instance that can be imported from eth_keys.keys which will lazily use whatever backend has been configured via environment variable.
• Signature.recover_public_key_from_msg and Signature.recover_public_key_from_msg_hash
• PrivateKey.sign_msg
• PublicKey.recover_from_msg and PublicKey.recover_from_msg_hash

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