feat: std::hint::black_box function.

[aakoshh]

Description

Problem*

Resolves #6471

Summary*

Added a std::hint Noir module with a black_box builtin function that is treated during SSA optimisation steps as if it could potentially have side effects, which prevents the value it is applied to from being simplified to a known value and the instruction from being removed.

Additional Context

Using a builtin rather than foreign so that this can be called from non-Brillig code.

I added an Intrinsic::Hint because Intrinsic::BlackBox was already taken; perhaps Intrinsic::AsWitness could be moved into Hint as well (not that it matters, but the comments call it a hint too).

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]

Can you add some tests to check how it handles arrays, etc.?

[aakoshh]

Good shout, the slice tests uncovered a bug.

[aakoshh]

Added some tests with mutable references but that doesn't work very well. I left some comments about scenarios that didn't work out, let me know if you think there's anything we can do about them.

[TomAFrench]

I'd like to take another look at this today after scrum

[aakoshh]

@TomAFrench have you had a chance to look at this?