Moves into borrowck

Move the computation of what data is moved out of `liveness` and into `borrowck`. The resulting code is cleaner, since before we had a split distribution of responsibilities, and also this avoids having multiple implementations of the dataflow code. Liveness is still used to report warnings about useless writes. This will go away when we get the control-flow graph code landed (working on that).

Also adds borrow checker documentation.

Fixes #4384.
Required to support once fns and to properly fix closures (#2202).
First step to generalize our treatment of moves somewhat as well.