Implement Decimal support for Postgres backend

[GregoryTravis]

Pull Request Description

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• [ ] The documentation has been updated, if necessary.
• [ ] Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• [ ] All code follows the Scala, Java, TypeScript, and Rust style guides. In case you are using a language not listed above, follow the Rust style guide.
• [ ] Unit tests have been written where possible.

[GregoryTravis]

Not ready to merge this yet -- I want to add many more tests checking for accidental Decimals.

[GregoryTravis]

Ready to merge; returns types verified.