Generator for pointers (PtrTo and Ptr) do not shrink pointer to nil

[steffnova]

Ptr generator can generate either nil pointer or pointer to a type defined by it's "element" generator:

// This will generate either <*int>(nil) or <*int>(10) (or any other int value)
generator.Ptr(generator.Int())

However Ptr doesn't provide shrinker that can shrink pointer to nil. (it only shrinks the value that it points to).

PtrTo generator on the other hand will always generate a non-nil pointer.

It makes sense to merge these two generators into a single generator and add aforementioned shrinking capabilities.