MaryValue fixes

[lehins]

Description

This PR split into 3 major commits (thus it probably would be easier to review commit by commit)

• improves MaryValue generators
• fixes MaryValue deserialization. It reverts back the enforcement of positive ADA in the MaryValue back to non-negative ADA. This must be the case due to possibility of zero ADA in ValueNotConservedUTxO.
• Changes type for ADA in MaryValue from Integer to Coin

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

[TimSheard]

Looks good to me. The only thing that looks strange is the missing EncCBOR instance.

[teodanciu]

Looks good to me