Field Instance Permissions

This is mostly blatantly stolen from Field permissions. The change is that I try to make the module define permissions per-instance, rather than per-field.

The idea is to work completely separately from field_permissions, and not to interfere with it, so both modules can be in use at the same time. This module can be later merged to the original module, but I am not sure if there is much interest in that. See #1221186: Bundle based Field permissions.

The module is implemented, but not tested very well at the moment.