Handle priority in the lints table

[suaviloquence]

Like the cargo [lints] table, the priority key in a lint entry is optional (defaults to 0), and a lower (more negative) priority overrides a higher (more positive) entry that configures the same lint (which lets us define how to handle a lint group containing a lint, and a lint group). Changing OverrideStack from holding Arc<OverrideMap>s to just OverrideMaps is setting up for the upcoming lint group PR where each OverrideMap will be compiled based on lower items in the OverrideStack, so shared ownership doesn't make sense to use anymore.

[obi1kenobi]

This looks great.

In a subsequent PR, could you think about a way to warn users if a lint could hit multiple configs that affect it with the same priority level? That's analogous to this clippy lint for regular cargo lints.

If you can't think of an easy and elegant way to do it, it's fine to just open an issue about it and move on — we shouldn't let it block other work if it's more than a 15min thing to build.