feat(acir_gen): Brillig stdlib

[vezenovm]

Description

Problem*

Expands usage of #3907 to our Brillig directives.

Summary*

The Brillig stdlib differs slightly from normal Brillig functions calls. We can insert the generated Brillig bytecode at any point during ACIR gen. This includes within the GeneratedAcir struct itself. We have a few requirements on what we want to achieve:

• We do want to have to thread the ACIR gen SharedContext that is used for generating normal Brillig function pointers to the GeneratedAcir. Why would we want a reference to code generation inside of our Generated ACIR structure (we don't)?
• We want to maintain one Brillig stdlib for an entire program. We do not want to change the BrilligCall opcode so we need them to be a part of the main list of unconstrained_functions in a program.
• Function IDs reference a flat list, so reserving some number of slots would be wasteful if the program does not eventually use a Brillig stdlib function in that slot.

So instead we maintain a map as part of the GeneratedAcir that notes when we at which opcode location we inserted a call to a BrilligStdlibFunc. The ID at this point will just be 0. After finishing ACIR generation for a function, only then do we resolve the IDs for these BrilligCall opcodes.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Some initial thoughts.

[jfecher]

LGTM