Add assert_is_exclusive_system function

[Nilirad]

What problem does this solve or what need does it fill?

We already have assert_is_system, but it does not work with systems that take &mut World.

What solution would you like?

A function called assert_is_exclusive_system that does nothing if the argument implements IntoExclusiveSystem, and fails to compile otherwise.

What alternative(s) have you considered?

Alternatively, you can try to do this:

bevy_ecs::system::IntoExclusiveSystem::exclusive_system(my_exclusive_system);

but it's ugly, and it's not symmetric with assert_is_system.

Another alternative is to wait for #4166 to be merged, which would deprecate exclusive systems as a special type of system.

Additional context

• The body of the function should be really empty, not like the if false crime that has been committed against assert_is_system.
• The function should have an #[allow(UnusedVariables)] attribute, otherwise we would have to prefix underscore the parameter (ugly).

[DJMcNab]

Surely we should just wait for #4166 (or well, an updated version addressing the concerns)?

[Nilirad]

I added the PR link to the alternatives.

That being said, I think this can stay open for newcomers that want to do their first PR. Or maybe not, for a change that will get reverted soon. I don't know, I'll leave it up to the maintainers.

[alice-i-cecile]

My feeling is that #4166 is preferred, but I would probably vote in favor of merging a PR that added this, since it should be easily caught during the refactor.

[Nilirad]

Is #4166 going to be merged before 0.8? Alternatively this would be a rather trivial change that can easily land on the next release.

[alice-i-cecile]

I think that's unlikely to be a high enough priority; I would be happy to merge a tiny PR adding this.

[Nilirad]

Fine. I'm going to mark this as good first issue, since its implementation can be literally based on assert_is_system.