Compile multiple run conditions into one

[JoJoJet]

Objective

When multiple run conditions are attached to a system, they should be compiled together as one function.

Solution

Add a variant of System{Set}Config that preserves the types of run conditions and combines them with any newly-added ones. This allows the compiler to inline them into one function.

This optimization is performed opportunistically -- run conditions that have already been boxed and type erased are not combined, as there is no benefit to doing so. This means that structs that perform type erasure such as SystemConfig present a barrier for this optimization. Resolving this should be possible in the future, but it would require a much higher-impact change that involves preserving the type data for all SystemConfig instances.

[alice-i-cecile]

I don't love adding more complexity here, but the aim is valuable. Great comments!

Requests:

1. Add a matching Either combinator (to avoid mangling Or from queries).
2. Add doc tests to both Either and And.
3. Add links to both Either and And from the Condition docs. not from [Merged by Bors] - Add condition negation #7559 would be nice to mention there too.

[JoJoJet]

1. Add a matching Either combinator (to avoid mangling Or from queries).

I'm assuming you want this in order to satisfy #7584 -- however the And combinator I've added here intentionally doesn't short-circuit, so it won't resolve that issue. Instead of building on the And type by adding Either, I think it would be better to keep And an implementation detail and make it #[doc(hidden)].

I am working on another PR that would satify #7584, though.

[alice-i-cecile]

Okay great, I'm fine to cut 1 from this PR :)

[JoJoJet]

I'm taking this off of the milestone if that's okay, since 0.10 is so close and this is just a nice-to-have optimization.

[JoJoJet]

Closing this since it's out of date, and I don't consider this to be high enough priority to get it working again.