Remove the implicit `'static` bound on `Send`.

Previously Send was defined as `trait Send: 'static {}`. As detailed in
https://github.com/rust-lang/rfcs/pull/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 #22251.

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