Support moving a cell to a new ownership

[uazu]

TLCell already supports a kind of transfer of ownership, but only between threads, due to there being an owner in each thread.

A QCell-like cell could potentially support transferring ownership since ownership is determined by the key value stored in the cell. However that key value is immutable. To make it mutable means either using an atomic type or else maybe by making the cell non-Sync and using a plain Cell to contain the key value.

The proposed address-based-key cell (issue #14) could also support transferring ownership to some other owner's address in a similar way, if there was an ID type that could be used to pass the other owner's address. (We don't need &mut on the second owner in order to transfer ownership to it, just the address.)

The other cells (TCell, TLCell and LCell) can't support transferring ownership because ownership is hardcoded at compile-time and is checked by the compiler, and so cannot be modified at runtime.