Split HIT files into Core and lemmas

[Alizter]

We should split the various HIT files like GraphQuotient, Coeq, Pushout into separate files like we do for Join which will make the compilation faster.

@jdchristensen What about splitting the various HITs into files like we do for Join? That way we can have the core definition and various results like functionality or flattening in separate files? In any case, I would suggest doing the splitting later when we have the flattening lemmas and we can do it all in one go.

Originally posted by @Alizter in #1851 (comment)

[jdchristensen]

Do you mean creating subdirectories for each type of colimit, like Colimits/GraphQuotient/*, Colimits/Coeq/*, Colimits/Colimit/*, Colimits/Pushout/*, etc? That might be ok, but it might also be overkill, since currently the only thing we separate are the flattening lemmas.

[Alizter]

@jdchristensen Yes that is what I meant. I'm fine with splitting the flattening lemmas into separate files too. One thing to keep in mind is that the flattening lemams are now much lighter so it might not be as big of an issue as before with the colimits library. I think we will have to see once those PRs are merged.