Rollup merge of #120321 - Nadrieril:cleanup-cx, r=compiler-errors

pattern_analysis: cleanup the contexts

This cleans up a bit the various `*Ctxt`s I had left lying around. As a bonus this made it possible to make `PatternColumn` public. I don't have a use for that yet but that could come useful.

`UsefulnessCtxt` looks useless right now but I'll be adding a field or two in subsequent PRs.

r? `````@compiler-errors`````

Author: Nadrieril