Split ValueBalance methods into NegativeAllowed and NonNegative

[teor2345]

Motivation

Some ValueBalance methods only make sense for NegativeAllowed or NonNegative.

If we make them generic, it's harder to use them correctly.
And type inference can fail on the generic, which makes code harder to write.

Solution

• split out some methods into ValueBalance<NonNegative> and ValueBalance<NegativeAllowed>
• fix serialization tests to use ValueBalance<NonNegative>, like the finalized state
• fix an unused import clippy lint

Review

@oxarbitrage can review this PR, it also includes some of the test methods from PR #2647 and oxarbitrage#130

Reviewer Checklist

• Code implements consensus rules
• Existing tests work

[oxarbitrage]

Looks good, thanks.