Cache subtypes to speed up selector dispatch

[epatters]

Addresses #1438 by caching the results of the subtypes function. This PR reduces the runtime of the Catlab docs build from ~1 hr to 10 min.

The approach taken here has the side effect that a selector cannot have more steps assigned to it after it has been dispatched at least once, but hopefully that is not a problem.

[KristofferC]

Unrelated to the PR, but this whole "Selector system" feels kinda odd to me. Using dynamic dispatch and reflection for what amounts to a "do this, then do this" construct seems quite complicated and slow.

[MichaelHatherly]

Using dynamic dispatch and reflection for what amounts to a "do this, then do this" construct seems quite complicated and slow.

Yeah, would be good to get rid of it. I guess it just depends on whether any users are actually making use of it for extension purposes. (Edit: from a brief gh search there doesn't appear to be any usage, so after this long not having it made use of we could safely remove it without much problem. If anyone who is using it sees this please speak up.)

[MichaelHatherly]

The approach taken here has the side effect that a selector cannot have more steps assigned to it after it has been dispatched at least once, but hopefully that is not a problem.

@epatters did you also try the initial approach you outlined in #1438 (comment)?

A simple workaround would move the call sort(subtypes(T); by = order) from inside dispatch to the body of the main function expand.

[epatters]

@MichaelHatherly, I was originally going to try that, but then I realized it would fragment the dispatch logic across the package. I thought it would be better to isolate the workaround where the problem occurs so that the rationale will be clear to readers of the code.

[mortenpi]

Does this kind of undo #620?

If anyone who is using it sees this please speak up.

DocumenterCitations.jl is an example that uses the selectors API for extending Documenter. But that shouldn't block us from improving it -- it's not, strictly, even part of the public API.

[MichaelHatherly]

DocumenterCitations.jl is an example that uses the selectors API for extending Documenter.

Cool package. Looks pretty active, so probably best not to disrupt it too much if possible.