Cleanup: ClientPointer and ClientField should implement a common interface

[rbalicki2]

Many functions in validate_client_field.rs should accept this interface, instead of the parts of client pointers/client fields.

This task can probably include renaming variables and the like in validate_client_field (which should be renamed validate_client_type) to say type instead of field/pointer.

Anyway, low-pri, just don't want to forget about it.