Add generic IStructuralEquatable<T> and IStructuralComparable<T>

[charlesroddie]

Currently there is a non-generic IStructuralEquatable. This causes 1. loss of type safety, including requirements for implementing code to downcast, 2. poor performance especially with structs.

So generic IStructuralEquatable and IStructuralComparable should be added.

There is good progress towards obsoletion of non-generic non-structural equality and comparison, with the deprecation of collection classes that use them: non-generic collections shouldn't be used

A similar approach should be taken for IStructuralEquatable and IStructuralComparable.