Mc/easy private uint

[iAmMichaelConnor]

Description

Example of a private uint state with add and subtract methods. No notes. No sets.

Often developers will need the low-level control over utxos. But if they don't, this might be a nice state variable to provide in our library.

Currently no typescript tests. But I should probably get back to the other things on my todo list, and stop playing with Noir Contracts :)

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• The branch has been merged or rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.