util: add CloneBoxService

[davidpdrsn]

This upstreams a little utility I'm using a bunch in axum. Its often
useful to erase the type of a service while still being able to clone
it.

BoxService isn't Clone so previously you had to combine it with
Buffer but doing that a lot (which we did in axum) had measurable
impact on performance.

Note this doesn't add a !Send variant. I personally have no use
for that but if someone thinks it would be nice I can do a follow up PR.

[hawkw]

Overall, this looks great to me --- we may also want to add methods to ServiceExt and/or ServiceBuilder for wrapping a service in CloneBoxService, but we can do that in a follow up PR.

I commented on a few minor things. The only one that I think is actually important to address before merging is the unnecessary fmt::Debug bounds on CloneBoxService's type parameters: since those types are never formatted in the Debug impl, it shouldn't be necessary for them to implement Debug.

Beyond that, this looks good to me and I'll be happy to approve it once that's addressed!

[hawkw]

take it or leave it: we may want to consider including the request and response type names here, so different instances of CloneBoxService can be distinguished? Maybe something like

Suggested change

	fmt.debug_struct("CloneBoxService").finish()
	write!(
	fmt,
	"CloneBoxService<Request = {}, Response = {}, Error = {}>",
	std::any::type_name::<T>(),
	std::any::type_name::<U>(),
	std::any::type_name::<E>(),
	)

but, on the other hand, this could be quite long, and we might want to correctly handle multi-line formatting in alt mode ({:#})...so, maybe that's something to hold off on and add later if it's needed...

[hawkw]

also, BoxService doesn't do this, so it's probably better left for a separate PR if we do want to add something like that...

[davidpdrsn]

I was actually thinking the type params are there such that this could be improved in the future, without a breaking change. I think I'll leave it as it is now and make an issue about improving the formatting, for BoxService as well.

[hawkw]

I was actually thinking the type params are there such that this could be improved in the future,

I don't think we want the type params to be Debug in any case; we can't actually include values of those types in the formatted output, since they're not part of the struct. I think the where clause won't ever be useful for future improvements to the debug format, since we'll never have instances of those types to format when formatting a CloneBoxService instance. We can use type_name to get the names of those types without requiring those types to be Debug.

[davidpdrsn]

Oh yeah doh of course! I've removed the bounds.

[davidpdrsn]

@hawkw I think adding more convenience methods to ServiceExt and ServiceBuilder for this is a good idea. I'll do that in a follow up PR.

[hawkw]

okay, this looks good to me! thanks!