Allow traits to declare unconstrained functions

[nventuro]

Problem

Currently all trait functions must be constrained: adding the unconstrained keyword results in a parser error. It'd be good to also be able to make these functions unconstrained, as there are many scenarios in which we require this property from an interface.

An example is Aztec's note abstraction, which includes the ability to compute a note nullifier. These nullifiers include a secret that is injected by an oracle and then constrained by an external component. More concretely, we take a mutable reference to an array of validation requests that are constrained to later be fulfilled - this deferring exists because said constraining must be done by a different circuit.

However, we also need to have an unconstrained way of computing these notes, in which we essentially skip all of the validation. Due to these complicated interactions with the deferred validation requests, we've not been able to come up with a better abstraction than to simply have a second function that does not perform any constraining - the obvious problem being that we cannot mark this function as unconstrained, even though it is.

This may not be a huge deal today, but with #4442 there'll be an expectation of being unable to accidentally call unconstrained functions from constrained contexts, and lacking the ability to properly tag trait functions as such prevents us from leveraging this feature.

Happy Case

trait Foo {
  fn bar() -> Field;
  unconstrained fn bar_unconstrained() -> Field;
}

And then force implementations of the trait to make bar_unconstrained be indeed unconstrained.

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

None

Blocker Context

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response