This library defines the PinCell type, a pinning variant of the standard
library's RefCell.
It is not safe to "pin project" through a RefCell - getting a pinned
reference to something inside the RefCell when you have a pinned
refernece to the RefCell - because RefCell is too powerful.
A PinCell is slightly less powerful than RefCell: unlike a RefCell,
one cannot get a mutable reference into a PinCell, only a pinned mutable
reference (Pin<&mut T>). This makes pin projection safe, allowing you
to use interior mutability with the knowledge that T will never actually
be moved out of the RefCell that wraps it.