channel: Add try_recv and deprecate try_next

[cmrschwarz]

Adds try_recv and deprecates try_next as discussed in #2936.

It does so by changing the previously private TryRecvError internals from a struct to an an enum.

Note that this is technically not semver compatible with 0.3 as the following is legal (albeit nonsensical) Rust:

let TryRecvError {..} = my_err;

This would no longer compile after this change.
I'm happy to introduce some slightly hacky TryReceiveError or similar instead if that is preferred.

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I've also moved most most tests over to try_recv but kept around these two using try_next just to make sure it isn't somehow broken.

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Adding recv aswell proved harder than expected because I can't add a dependency on
futures-util, which would be cyclic. I see two options:

1. Take out the futures-channel specific code from futures-util and move it into futures-channel.
2. Essentially reimplement Next as Recv. Might be reasonable as Next is pretty small.

Which would you prefer? I'm happy do do either as a follow up if that still seems good, or of course do a third option if there's a better Idea.

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Closes #2936
Closes #2197

[cmrschwarz]

Sorry for the double CI, I accidentally left a comment that doesn't make sense anymore. Fixed.

[taiki-e]

Thanks!

As for recv, I think it is reasonable to copy the code of Next from futures-util. It is small enough.