Prefer System Tuples, replace in_schedule, aggressive add_systems merging

[cart]

Followup to #9 (which should be merged first)

This pr includes a number of small UX tweaks (some of which will be controversial), each split up into its own commit. I personally like where it landed, but every commit is negotiable. The last couple are especially controversial I think 😄

[alice-i-cecile]

I like add_systems_to. The double-parantheses and tuple abuse are a bit naff, but we'll need a builder style escape hatch anyways to account for arbitrary lengths so people who prefer it can use that instead.

add_system should still exist, but I do like grouping systems by default.

Tuple nesting seems both natural and good.

[maniwani]

LGTM.

I'd prefer if each sentence was on its own line to be consistent with the rest of the doc (makes it easier for GitHub to diff changes).

[cart]

The double-parantheses and tuple abuse are a bit naff, but we'll need a builder style escape hatch anyways to account for arbitrary lengths so people who prefer it can use that instead.

Cool cool. I made sure to call out Into<SystemCollection> to leave the door open to arbitrary builder apis.

add_system should still exist, but I do like grouping systems by default.

Yeah I think I'm on that team too (easier migration path, reads better for single systems). I intentionally took it "too far", just too see how it made us feel 😄

I'd prefer if each sentence was on its own line to be consistent with the rest of the doc (makes it easier for GitHub to diff changes).

I haven't sorted out my feelings on this style, but I'm happy to align with it here for consistency.

[maniwani]

I haven't sorted out my feelings on this style, but I'm happy to align with it here for consistency.

It should render the same. I just mean in the file.

If you have two sentences on different lines, the generated Markdown will still show them on the same line.

My first sentence.
My second sentence.

My first sentence. My second sentence.

They would only be rendered on different lines when there was an additional newline in between them, or if there were two spaces after the first sentence. Not what I wanted.

Having each sentence on a different line of the file has made it a little easier for us to view the changes in the diff view. Though I guess if you're going to merge the RFC and we never edit this document again, then it doesn't really matter.

[cart]

It should render the same. I just mean in the file.

Yeah I'm not worried about that. My only hangup is "forcing people into this style as a matter of policy". Idk if the churn there is worth the diff-reviewability wins. But thats a conversation we should have later. Happy to align with the style in the current RFC. Consistency is the only reason I need.

[JoJoJet]

Renaming SystemLabel -> SystemSet will clear up a lot of potential confusion, I think. Removing some macros is nice, too.