sync: add mpsc::Sender::{reserve_owned, try_reserve_owned}

[hawkw]

Motivation

The mpsc::Sender::reserve method currently returns a permit that borrows
from the Sender. It would be nice to have a version of it that returns
an owned permit.

Solution

This branch adds an OwnedPermit type and Sender::{reserve_owned, try_reserve_owned} methods. Unlike the comparable methods on
Semaphore, these methods do not require an Arc<Sender> as the
receiver; this is because the sender internally reference counts the
channel and is already cheap to clone. Requiring an Arc would simply
add an unnecessary second layer of reference counting, which is not
ideal; instead, the documentation encourages the user to clone the
sender prior to calling reserve_owned when necessary.

Since these methods take the Sender by value, they also have the ability
to return the Sender from a successful OwnedPermit::send. This
allows them to be used without additional clones. Essentially, this is a
very simple type-level encoding of the sender's state, with the
transitions

      ┌──────┐
 ┌───►│Sender├───┐
 │    └──────┘   │
send          reserve
 │ ┌───────────┐ │
 └─┤OwnedPermit│◄┘
   └───────────┘

Additionally, I added an OwnedPermit::release, which returns the
Sender and releases the permit without sending a message.

Closes #3688

[hawkw]

Honestly, most of the time I spent on this was trying to write docs that actually express how this should be used --- the implementation was pretty easy. Hopefully the docs are helpful for new users...

[Darksonn]

Thanks! I have some comments regarding the documentation. The implementation looks good.