Replace use of point and scalar defaults

[AaronFeickert]

The codebase sometimes uses the Default trait to get either the identity point of the Ristretto group (in either compressed or uncompressed form) or the zero scalar. This is fine in practice due to the implementation of that trait on the relevant types. However, it seems non-idiomatic and has a bad code smell.

It would be more idiomatic to use the Identity trait implementation for Ristretto types and the ZERO constant for scalars. This makes much more unambiguous what the intent is.