Prepare Tokio v1.12.0

[Darksonn]

1.12.0 (September 21, 2021)

Fixed

• mpsc: ensure try_reserve error is consistent with try_send (#4119)
• mpsc: use spin_loop_hint instead of yield_now (#4115)
• sync: make SendError field public (#4097)

Added

• io: add POSIX AIO on FreeBSD (#4054)
• io: add convenience method AsyncSeekExt::rewind (#4107)
• runtime: add tracing span for block_on futures (#4094)
• runtime: callback when a worker parks and unparks (#4070)
• sync: implement try_recv for mpsc channels (#4113)

Changed

• macros: run runtime inside LocalSet when using macro (#4027)

Documented

• docs: clarify CPU-bound tasks on Tokio (#4105)
• mpsc: document spurious failures on poll_recv (#4117)
• mpsc: document that PollSender impls Sink (#4110)
• task: document non-guarantees of yield_now (#4091)
• time: document paused time details better (#4061, #4103)