feat: Allow arbitrary noir functions to be unconstrained

[jfecher]

Related issue(s)

Resolves #

Description

Summary of changes

• Renames unsafe back to unconstrained
• Allows any noir function (not just contract functions) to be unconstrained
• Renames ContractVisibility to ContractFunctionKind
• To keep the JSON ABI the same, the ContractFunctionKind that is outputted still contains the three possible values of 'open', 'secret', or 'unconstrained' as mutually exclusive. Despite the fact the frontend now allows functions that are both 'open' and 'unconstrained' as a side effect of them being separated.

Dependency additions / changes

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Documentation needs

• This PR requires documentation updates when merged.

Additional context

[critesjosh]

@jfecher should all instances of the unsafe keyword be updated to unconstrained in the docs?

I think the only place we mention it is on the "Zeroed Function" page, std::unsafe::zeroed. Is it still in stdlib::unsafe?

[jfecher]

@critesjosh I don't believe we've ever had an unsafe keyword in Noir. The unsafe in std::unsafe::zeroed is just the name of the module so it doesn't need to be changed - it's still at that location.