Move checking for moves and initialization of local variables and patterns into
borrow checker and generalize what moves are allowed. Fixes a nasty
bug or two in the pattern move checking code. Unifies dataflow code
used for initialization and other things. First step towards
once fns. Everybody wins.

Fixes #4384. Fixes #4715. cc once fns (#2202), optimizing local moves (#5016).
Update docs on the formal basis of the borrow checker.
Reconcile docs and code, adding examples and adding RESTR_CLAIM
Reconcile treatment of &mut with the docs
Update documentation for moves