Migrate to std::sync::TaskPool

No need to have our own bundled implementation when the standard library
suffices!
Remove lots of no longer necessary + Send annotations
auto merge of #1024 : alexcrichton/cargo/less-send, r=brson
auto merge of #1022 : alexcrichton/cargo/std-thread-pool, r=brson

No need to have our own bundled implementation when the standard library
suffices!
Use rust-installer for installation

This is just a refactoring to put Cargo and Rust on the same installer script.
auto merge of #1034 : brson/cargo/rust-installer, r=alexcrichton

This is just a refactoring to put Cargo and Rust on the same installer script.
Sleep before unlinking to fix a flaky test on windows