feat: Shielding and unshielding token contract

[LHerskind]

Extend the non-native noir token contracts such that it is possible for a user to "shield" or "unshield" his funds.

unshield being that funds are moved from notes in balances, the public_balances.

shield being that funds are moved from the public_balances to notes in balances.

A successful implementation should include at least tests where:

• User deposits privately, then unshields part of the assets and withdraw from public
• User deposits publicly, then shields part of the assets and withdraw from private

Consider that shielding can happen two ways, it can be a 1-step action, where the new note is inserted directly which leaks the new owner. Or it can be a 2-step action where a note is inserted and then later claimed for spending. Better privacy but worse UX.

Consider looking into the partial claim notes such that shielding can be done using these, making it a 1-step action that does not leak the owner.

[rahul-kothari]

Fixed in #915