Introduce the concept of a public output

[kevaundray]

Problem

Currently we have the concept of a public input, and we have the concept of a "return" field in the public inputs which functionally serves as public output.

Solution

We can:

• Add an optional return field in the Abi which signifies the output
• Add a public outputs vector in ACIR to signify public witness values which are semantically output

The second point is blocked until we complete the ACIR migration

Alternatives considered

Additional context

• This cleans up some of the code in the package manager as a side effect; because ACIR does not have the concept of a public output, nargo needs to ensure that when we pass public input values, we do not have any duplicates. This duplication can happen when a public input is set as a public output.

If we add public outputs to ACIR, we can by proxy add logic to deduplicate public input/output indices

• SSA setpub instruction will then add to the public_outputs field in ACIR instead of the public inputs field

[TomAFrench]

Going to transfer this to the ACVM repo as the knock-on effects on deduplication of public inputs is handled as of noir-lang/noir#862