Previously Send was defined as `trait Send: 'static {}`. As detailed in
rust-lang/rfcs#458, the `'static` bound is not
actually necessary for safety, we can use lifetimes to enforce that more
flexibly.

`unsafe` code that was previously relying on `Send` to insert a
`'static` bound now may allow incorrect patterns, and so should be
audited (a quick way to ensure safety immediately and postpone the audit
is to add an explicit `'static` bound to any uses of the `Send` type).

cc rust-lang#22251.

In most places this preserves the current API by adding an explicit
`'static` bound.

Notably absent are some impls like `unsafe impl<T: Send> Send for
Foo<T>` and the `std::thread` module. It is likely that it will be
possible to remove these after auditing the code to ensure restricted
lifetimes are safe.

More progress on rust-lang#22251.

Per RFC 458.

Closes rust-lang#22251.

The lifetime was previously, incorrectly unconstrained.