199 verify signature

[bartossh]

• Signature can verify itself against the public key.
• Signature precalculates digest itself again to check if the message is not corrupted before validating the signature.
• I've introduced new functions to get the public key and private key in the form of EVP_PKEY (from openssl/evp.h) for verification. This will allow for future development when transactions will have a bytes buffer of raw public key encoded to base58. So we know that the issuer with the given address is the owner of a signed message.

Please test on your machine with make memcheck <- to verify against memory leaks.
Test cleanup with make clean.

[kmroz]

👍🏻