[#18] Implement secp256k1 and P256 crypto to full extent

Description

Problem: Currently there are secp256k1 and p256 crypto for working with tz2 tz3 addresses, but they are not implemented fully.

secp256k1 lacks proper decoding and p256 does not have proper implementation all together.

Solution: Implement the missing parts of secp256k1 and p256 so that they can be used to do signature checking properly.

Related issue(s)

Resolves #18 (closed)

Depends on #267 (closed)

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