Remove our own error types from crossbeam-queue

[taiki-e]

Currently, crossbeam-queue has two error types: PopError, PushError.

• PopError is a unit struct with no field, and is returned by ArrayQueue::pop and SegQueue::pop if the queue is empty.
• PushError is a tuple struct with a public field that stores the element that user tried to push, and is returned by ArrayQueue::push if the queue is full.

I guess the current API was influenced by channels, but there is only one kind(variant) of error in each, and unlike channels, multiple kinds(variants) of errors do not occur.
And, neither of these provides additional information about the error (fields are all public, so it doesn't seem like they are going to provide them in the future).

So, APIs that return Option (on pop methods) and Result (on ArrayQueue::push) without our own errors are simple, and I think it is preferable.
(Especially for the pop methods, it will be more consistent, because all other public pop methods in crossbeam return Option.)

(Of course, this is a breaking change, but the master already has some breaking changes.)

[ghost]

This seems like an okay change to me. :) I only chose to return Result from pop() for symmetry between push() and pop().

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci