Add Atomic::compare_exchange(_weak) and deprecate Atomic::compare_and_set(_weak)

[taiki-e]

Instead of changing the argument of Atomic::compare_and_set(_weak), add new methods Atomic::compare_exchange(_weak) that always use two orderings.

This PR adds:

• Atomic::compare_exchange
  Unlike compare_and_set, this always receive two orderings.
• Atomic::compare_exchange_weak
  Unlike compare_and_set_weak, this always receive two orderings.
• CompareExchangeError (error returned by Atomic::compare_exchange(_weak))
  The definition is the same as CompareAndSetError, just renamed CompareExchangeError.

This PR deprecates:

• Atomic::compare_and_set
• Atomic::compare_and_set_weak
• CompareAndSetError (error returned by Atomic::compare_and_set(_weak))
  This is now a type alias of CompareExchangeError.
• CompareAndSetOrdering

Closes #621

[taiki-e]

I don't know why this method was originally called compare-and-set, but Wikipedia says a variant of compare-and-swap that returns a boolean indicating whether a substitution was performed is often called compare-and-set. https://en.wikipedia.org/wiki/Compare-and-swap

The result of the operation must indicate whether it performed the substitution; this can be done either with a simple boolean response (this variant is often called compare-and-set), or by returning the value read from the memory location (not the value written to it).

As this method actually returns a value rather than a boolean, I think it's okay to call it compare_exchange. (Given that other similar methods in the standard library and crossbeam are called compare_exchange, it's probably more consistent.)

[jeehoonkang]

std's compare_exchange receives two orderings: one for success and the other for fail cases. But this PR's compare_exchange receives only one that can be rendered into two. Do you think we need to match them?

[taiki-e]

But this PR's compare_exchange receives only one that can be rendered into two.

Uh, both methods (compare_exchange(_weak)) added by this PR always receive two orderings, not CompareAndSetOrdering.
I guess what you are mentioning is the behavior of the methods (compare_and_set(_weak)) that will be deprecated by this PR.

crossbeam/crossbeam-epoch/src/atomic.rs

Lines 456 to 465 in 4f0fcd5

	pub fn compare_exchange<'g, P>(
	&self,
	current: Shared<'_, T>,
	new: P,
	success: Ordering,
	failure: Ordering,
	_: &'g Guard,
	) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
	where
	P: Pointer<T>,

crossbeam/crossbeam-epoch/src/atomic.rs

Lines 531 to 540 in 4f0fcd5

	pub fn compare_exchange_weak<'g, P>(
	&self,
	current: Shared<'_, T>,
	new: P,
	success: Ordering,
	failure: Ordering,
	_: &'g Guard,
	) -> Result<Shared<'g, T>, CompareExchangeError<'g, T, P>>
	where
	P: Pointer<T>,

[taiki-e]

Updated the PR description to explain the changes in more detail.

[jeehoonkang]

bors r+

[bors]

Build succeeded:

• ci