correct handling of moves for struct-record update.

Part of #3678.  Fixes #2828, #3904, #4719.

Revert "librustc: Make unqualified identifier searches terminate at the nearest module scope. r=tjc"

This reverts commit a8d37af2.

Sadly, there's only one file that requires deprecated modes
(stackwalk)... So, forbid them everywhere else.

This removes most explicit uses of the + argument mode. Pending a
snapshot, I had to remove the forbid(deprecated_modes) pragma from
a bunch of files. I'll put it back!

+ mode still has to be used in a few places for functions that get
moved (see task.rs)

The changes outside core and std are due to the to_bytes trait and
making the compiler (with legacy modes on) agree with the libraries
(with legacy modes off) about modes.

Also:
- report illegal move/ref combos whether or not ref comes first
- commented out fix for #3387, too restrictive and causes an ICE

This involved some other changes:
- add a managed<T> wrapper that makes send_map usable from @-data
- implement map<K,V> for managed<send_map>

Unit tests are coming.