chore(Topology/Category): make TopCat an abbrev

[Vierkantor]

Like we did for the categories of rings in #11595 and #17612, turning TopCat into an abbrev allows us to get rid of some duplicate instances, and thereby reduce quite a few erws to simple rws.

There is one mild regression, see commit c051010: one _ had to be specified in slightly more detail before Lean could figure out what we mean. I believe the improvement in unification elsewhere weighs up to the regression, so I didn't check exactly what changed there.

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Note that this will conflict somewhat with the fixes in #17638 so please do not bors them together. They should be fine to merge in either order, though.

[github-actions]

PR summary 560253e688

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

- concreteCategory
- instance : CoeSort TopCat Type*

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

[Vierkantor]

Nevermind, looks like all these fixes are actually coming downstream from #17583, so they should be part of #17638 instead.