Remove Times trait

`Times::times` was always a second-class loop because it did not support the `break` and `continue` operations. Its playful appeal was then lost after `do` was disabled for closures. It's time to let this one go.
auto merge of #11672 : bjz/rust/remove-times, r=brson

`Times::times` was always a second-class loop because it did not support the `break` and `continue` operations. Its playful appeal (which I liked) was then lost after `do` was disabled for closures. It's time to let this one go.
move concurrent stuff from libextra to libsync
solve import error on workcache + change doc title
Merge branch 'move-extra-sync' of github.com:JeremyLetang/rust into move-extra-sync