Remove mfio_assume_linear_types

[h33p]

Pinned objects in rust include a drop guarantee, meaning, we are overly conservative with the use of stack allocated buffers.

In 95238ce, we resolved (as best as currently possible) the safety problems of sharing pinned data across threads.

The next step would be to eliminate mfio_assume_linear_types config switch, update documentation, and ensure that we move the Future implementation on &'a Packet<Perms> to something where pinning must be done explicitly, ensuring !Unpin.

The !Unpin bit is the reason why we are delaying this work until 0.2.x series.