Support Ethereum secp256k1 signature verification

[ethanfrey]

Part of #751
Requires #780
Follow the same steps as in #752

Add a method that verifies a public key and signature in the format used by ethereum (compressed pubkey, RLP encoded signature). This can either be a separate verify_secp256k1_eth() method or provide a function to convert this into the same format used by cosmos and then verify it.

Note there is some pubkey recovery stuff going on with ethereum signatures: https://ethereum.stackexchange.com/questions/64380/understanding-ethereum-signatures

[webmaster128]

This needs

• secp256k1_verify Add secp256k1_verify API #780
• secp256k1_recover_pubkey Add secp256k1_recover_pubkey API #765
• contract side pre-hashing (keccack256) Add secp256k1_recover_pubkey API #765
• contract side DER decoder for signatures (see https://github.com/cosmos/cosmjs/blob/v0.24.0-alpha.22/packages/crypto/src/secp256k1signature.ts#L33-L74)

[webmaster128]

In #765 you find a full example how a contract can verify signed text. Extening this to transactions is now an exercise for the reader. You need DER decoding and RLP encoding in the contract.

[ethanfrey]

Very nice, that looks like a good design in #765 and the parsing of the message bytes should totally be left up to the contract