Enable prefetch iteration

[t4c1]

Enables iteration of prefetch atom to cover prefetch tile. In other words relaxes the requirement for the prefetch tile size to match prefetch atom size.

This is done by using the same path for prefetch that nvidia code uses - going through copy implementation.

This PR also:

• includes a lot of bugfixes for prefetch atom layouts
• adds some missing copy traits
• removes some duplicated prefetch implementations (_V atoms duplicating what is already in _N)

[joeatodd]

LGTM 👍