Optimize PromiseYieldExtensions.AwaitInstructionAwaiter

[timcassell]

PromiseYieldExtensions.GetAwaiter<TAwaitInstruction>(this TAwaitInstruction awaitInstruction) returns AwaitInstructionAwaiter<TAwaitInstruction> which needlessly queries the CancelationToken.IsCancelationRequested even when the user hasn't called .WithCancelation. This can be optimized by splitting out the cancelation logic to a separate AwaitInstructionWithCancelationAwaiter<TAwaitInstruction>.

This will be a breaking change, so it should wait for the next major version.

[timcassell]

While we're at it, we can also move the private Action _continuation; field out of the awaiter, and put it in a separate type when it's added to the update processor, to reduce the size of async state machines.