Refactor ThreadSafeBox to use non-optional type

[plemarquand]

Change the underlying storage type from Value? to storing Value, with optional-specific operations moved to constrained extensions.

This improves type safety by eliminating unnecessary optionality for non-optional types.

This stronger type constraint makes it more difficult for the struct to be misused. For instance in the current implementation the extension method increment() will fail to increment if the user forgot to initialize the ThreadSafeBox with a value. Now a value must be set initially before increment is called.

Also this patch prevents a race condition in the memoize methods where the call to the memoized body was not guarded by a lock which allowed multiple threads to call memoize at once and produce inconsistent results.

Finally, fix the subscript setter so it works properly with structs, as the if var value and then value set will set the value on a copy, not on the self.underlying struct. Now that the type is no longer optional this unwrapping isn't necessary and we can set directly on the value.

[plemarquand]

@swift-ci test

[jakepetroules]

We have an existing implementation for this; let's use it: https://github.com/swiftlang/swift-build/blob/main/Sources/SWBUtil/AsyncSingleValueCache.swift

[plemarquand]

This was to replace the async version of memoize that was removed from ThreadSafeBox, in order to avoid instantiating the AsyncCoordinator in every case.

AsyncCache does look like it can replace this use case, so I'll remove this and switch to it.

[jakepetroules]

AsyncSingleValueCache in this case, not AsyncCache (I edited my post after you already saw it)