Consider adding signed comparisons to `Prop`

We use only unsigned comparisons on the `Prop` level, even though at the `Expr` level there are both signed and unsigned comparisons.
Some expressions could be better expressed at the `Prop` level if we have signed comparison there as well.

Locate the definitions and usages of Prop and Expr, then compare the existing unsigned comparisons at the Prop level with the signed and unsigned comparisons at the Expr level. The work is complete when signed comparisons are available for Prop and the relevant behavior is covered by the project's existing checks.

Written by the indexing model from the issue text.