Merge iAssert and iPvsAssert.
To do so, we have introduced the specialization patterns: =>[H1 .. Hn] and =>[-H1 .. Hn] That generate a goal in which the view shift is preserved. These specialization patterns can also be used for e.g. iApply. Note that this machinery is not tied to primitive view shifts, and works for various kinds of goal (as captured by the ToAssert type class, which describes how to transform the asserted goal based on the main goal). TODO: change the name of these specialization patterns to reflect this generality.