Fix sums with negative RHS.

[TimSheard]

When solving for a variable (like 'N') that appears on the right-hand-side of a summation constraint like this

SumsTo (Right (Coin 1)) ₳ 15 <= ∑ sum Map{34 -> ₳ 5 | size = 15, sum = ₳ 125} + sum N

There is a problem with the current algorithm, if the type of the variable 'N' does not support negative values.
The current algorithm solves this by subtracting the the constants on the right (125) from the constants on the left (15), resulting in (-110) <= N, which has a solution N is at Least (-109). But since N cannot be negative we must settle for a solution N is at Least 0.

We add special code to consider this case. And also made a few error messages more precise.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

👍