wip: add get_validate_value_or_guard_async

[xldenis]

This PR implements a solution to #73, introducing a get_validate_or_guard_async method which allows passing a validation function to determine whether we should consider the currently inserted value as 'live'. If the value fails validation it will be replaced with a Placeholder and the guard returned.

todo:

• tests
• unsync versions
• non-async apis
• bench to confirm no perf regression (shouldn't be the case).

[xldenis]

I've been thinking about the semantics of this, and I think there's footgun in it currently. If multiple tasks call this api with different validation functions there's no guarantee that when you resume you have a value that will satisfy your validation function. This feels like it could easily lead to bugs, the only fix I can think of is that one of the waked tasks should get a placeholder and try inserting a new value.

[arthurprs]

Your concern makes sense. Hand-wavely, another option may be to hide the generic guard from the validation callers (e.g. GuardWithValidation), and write it in a way that it always tries to validate the value returned and loops/errors otherwise.