Allow eip712 signatures to use 712Domain as primaryType

[danfinlay]

As discussed here:
https://ethereum-magicians.org/t/eip-712-standards-clarification-primarytype-as-domaintype/3286

And merged in sig-util here:
MetaMask/eth-sig-util#51

Allows the creation of cheaper-to-verify signatures. Cheers to @SilentSicero

[danfinlay]

Oh we have to integrate this into our keyrings before this is a proper PR.

[danfinlay]

A brief rant:

The customary method of node-style dependencies (CommonJS require style) allows all dependencies to have their own versions of shared dependencies, which is great for making sure you don’t break dependencies, but can make a real pain out of managing a unified version bump across dependencies.

Ex: Bumping eth-sig-util requires bumping all of our eth-keyring modules up the dependency graph up to MetaMask:

An alternative approach, that I was discussing with @kumavis recently is treating dependencies like capabilities, in which case they are passed into dependent modules as constructor arguments. This means there’s more passing around modules, but it also means it’s easy to unify the behavior of many dependent modules in a complex graph, as well as potentially smaller bundle sizes. This is more like dependency injection, and makes dependency type-checking more important than perhaps more arbitrary semver strings.

An open question: Should I move our keyring controller behavior over to that pattern? Would make adding/revising signing methods easier in the future.