iterator: impl DoubleEndedIterator for adaptors
remove dead take glue code paths

Closes #7888
updated manual
PR #7637 followup: no need to print the set of removed files twice.
Reorganize README to make it more clear.

This also adds a note about required memory usage and instructions for
building from Git.
std: Remove old magic core mod
Added a new method to extra::future (unwrap) + a test
Merge pull request #7897 from thestinger/double

rollup of small pull requests

404de4f r=huonw
0cdb0a2 r=pcwalton
b082302 r=sanxiyn
b6a0138 r=huonw
8d64fa3 r=graydon
ddd8c15 r=thestinger
eb74f0c r=thestinger
Add Either::expect_{left,right}
Add Option::take_map{,_default}()
Remove redundant Atomic{Ui,I}nt types from unstable::sync
Add AtomicOption::fill() and AtomicOption::is_empty()
Reimplement ARC::unwrap() and friends.
Add UnsafeAtomicRcBox::try_unwrap()
Add KillHandle and implement exit code propagation to replace join_latch
Add kill::Death for task death services and use it in Task.
Remove join_latch
Add tests for KillHandle
Implement KillHandle::kill() and friends (unkillable, atomically). Close #6377.
Do a task-killed check at the start of task 'timeslices'.