auto merge of #8560 : kballard/rust/reserve-yield, r=pcwalton

Rename task::yield() to task::deschedule().

Fixes #8494.

diff --cc src/libstd/task/mod.rs
Simple merge

diff --cc src/test/run-pass/issue-2718.rs
Simple merge

diff --cc src/test/run-pass/task-comm-10.rs
Simple merge

diff --cc src/test/run-pass/task-comm-12.rs
Simple merge