feat: add unsafe blocks for calling unconstrained code from constrained functions

[TomAFrench]

Description

Problem*

Resolves

Summary*

This is an experimental PR to play around with unsafe blocks as a method to make it clearer when unconstrained values are entering the circuit to avoid under-constrained errors.

Calls to unconstrained functions will now fail to compile unless inside of an unsafe block. Note that any code inside of an unsafe block still lays down constraints as usual so it's possible to fully constrain the return values from any unconstrained function calls before returning a safe value out from the block.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] 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.

[github-actions]

🚀 Deployed on https://66bcbbf79028aea1fa537200--noir-docs.netlify.app

[TomAFrench]

This is going to need a decent amount of documentation added before it can be merged. I wanna get a temp check on this PR first however.

[TomAFrench]

We're only emitting a warning for now so this should be non-breaking afaik.

[jfecher]

I'm still undecided if we want to force users to write unsafe { ... } every time they call an unconstrained function. I'm aware of the dangers but it just seems very cumbersome to me. I'm not definitely saying "no" to this, I'm just a bit hesitant on it.

[TomAFrench]

Yeah, I'm a little hesitant on this as well tbh. I don't like the status quo where any function call could be spewing junk into your program but it seems like these unsafe blocks have a tendency to expand to cover most of the function.

[github-actions]

FYI @noir-lang/developerrelations on Noir doc changes.