Add TypedDataUtils.eip712Hash unit tests

[Gudahtt]

Unit tests have been added for TypedDataUtils.eip712Hash. These tests don't exhaustively check all possible inputs because there are a lot, and they're just passed along to hashStruct. So the hashStruct tests should catch any input-specific changes. Outside of behaviour internal to hashStruct, all of the behaviour should be covered by these tests.

Any calls to eip712Hash in the older signature tests have been removed, as they are now redundant.

[Gudahtt]

This depends upon #172

[rekmarks]

My initial impression here is the same as #170 (review), namely that these tests seem perhaps uselessly redundant. I'm not sure why it wouldn't suffice to check that it makes the expected calls to sanitizeData, hashStruct, and ethUtil.keccak given the possible branches inside the function, and rely on the encodeData tests for testing a wide range of inputs.

[Gudahtt]

This has been rebased to resolve conflicts

[Gudahtt]

I think these tests are mostly devoid of repeats actually! I only included a few sample inputs here. Most of it was focused on behaviour specific to this function, about how it encodes the type header and the data and concatenates them together, with the data being optional and omitted in some circumstances. And the peculiar range of expected inputs, since it will happily accept inputs that are contrary to the input types.

[rekmarks]

LGTM!