[new feature] verify signatures

[frankiebee]

New Feature Spec:

import { verify } from '@entropyxyz/sdk/utils/crypto'
import { Entropy, /*all the other things*/ } from '@entropyxyz/sdk'

.... // start up entropy

const sig = entropy.sign(args)
// these will have the same outputs they are the same functions
(await entropy.verify(sig) === await verify(sig))

if (await entropy.verify(sig)) {
// carry on
} else {
  throw new Error('Could not verify signature')
}

for signature interface see #434

function verify (Signature): Promise<boolean>

see also #437 for how we will handle cases where the program has it's own custom hashing algorithm