Feature: Add eth_signTypedData once it is stable

[ricmoo]

It may make sense to also include support for the EIP in the meantime using an experimental method name? Maybe in general there should be a way to indicate experimental and a version, e.g.

x201701_eth_signTypedData

and we promise to support any "x" prefixed method for at least 1 year.

See: https://github.com/ethereum/EIPs/pull/712#issuecomment-358750873