Add function keyE.

Give a value, keyE lifts the Key for that value into the query expression.

    keyE :: Esqueleto query expr backend =>
            Value (Key entity) -> expr (Value (Key entity))
Rename keyE to valJ and fix implementation.

As Felipe points out in the PR comments, the implementation can be
simplified to "val . unValue" but the type signature on the function
still prevents anything non-sensical happening.

See: https://github.com/prowdsponsor/esqueleto/pull/69
Merge pull request #69 from erikd/master

Add function valJ (was keyE).