Add crypto crate to the toolkit

[PumpkinSeed]

Based on the I609 I moved the original content to the crypto crate and make them public based on the pattern of the other crates.

Also changes on my side:

• Added the verify function to the secp256k1.
• Add tests for all of the public functions.

Leftover:
I didn't add ed25519 signing however the original ticket contains it. This requires a decision on your part, since I found a crate for that purpose, I would say do it in a different PR.
Also to make it consistent, currently the sign.rs only support secp256k1, I don't know whether we can use the same parameter-set for ed25529, but if yes we can create an enum for decide between algorithms to sign the data.

enum SignAlgorithm {
  Secp256k1
  Ed25519
}

[Cashmaney]

Thanks @PumpkinSeed! @reuvenpo can you take a look and make sure everything is rusty?

[reuvenpo]

You did not export the crypto subcrate from the main secret_toolkit namespace. I'll do it for you real quick

[reuvenpo]

awesome PR, thanks @PumpkinSeed :D