Add support for signTypedData_v4

[danfinlay]

UI should be identical to _v3, making implementation simple. The difference is it should use the newly merged v4 method in eth-sig-util@2.3.0, which supports arrays and recursive structs.

Created by our friends at Infura to support a new type of state channel they've designed!

Completing this issue also requires updating our docs site.

[BrendanChou]

@whymarrh @danfinlay Can you reopen this as the rpc call was not previously supported in eth-json-rpc middleware, so the extension itself still cannot process these calls.

I fix that with a PR here, but metamask-extension will still have to use the new npm version of eth-json-rpc-middleware MetaMask/eth-json-rpc-middleware#15

[danfinlay]

Merged your changed and published it as + eth-json-rpc-middleware@4.2.0

[danfinlay]

I'll get to this soon if you don't beat me to it.

[BrendanChou]

Hey Dan, is this going to be in 7.1.2? Should just be updating the eth-json-rpc-middleware package right?

[whymarrh]

@BrendanChou this should be out in 7.2.0 very soon, I'm going to bump this today