Support hint function declaration to bind with builtin function

[katat]

This is the first step to support hint function, by making use the builtin functions as hint functions.

Although it is not yet fully supporting the native hint function, this PR makes it able to type check the unsafe/hint attribute.

Usage:

Declares a hint function in a .no file to wire up with a builtin function under the same full qualified name.

hint fn calc(val: Field) -> Field;

It requires unsafe attribute when calling a hint function, otherwise the type checker will throw error. So the hint function caller has to use unsafe to acknowledge they are calling a hint function.

fn main(pub xx: Field) {
    let yy = unsafe calc(xx);
}

This can make the type checking ready for hint calculations before the native hint function support.
Once the hint function support is ready, we can just fill the hint function body in the native code and deprecate the hint builtin functions, while keeping existing hint function callers intact.

[mimoo]

LGTM, I'll have another pass but let's not wait to merge it!

[mimoo]

maybe more concise:

match (fn_info.is_hint, unsafe_attr) {
  (true, true) | (false, false) => (),
  _ => return Err(self.error(ErrorKind::ExpectedUnsafeAttribute, expr.span));
  }

[katat]

Actually there are two error types to handle:

noname/src/negative_tests.rs

Lines 476 to 500 in 18825a3

	let res = test_hint_builtin_fn(&qualified, invalid_code);
	assert!(matches!(
	res.unwrap_err().kind,
	ErrorKind::ExpectedUnsafeAttribute
	));
	}
	#[test]
	fn test_nonhint_call_with_unsafe() {
	let code = r#"
	fn calc(val: Field) -> Field {
	return val + 1;
	}
	fn main(pub xx: Field) {
	let yy = unsafe calc(xx);
	}
	"#;
	let res = tast_pass(code).0;
	assert!(matches!(
	res.unwrap_err().kind,
	ErrorKind::UnexpectedUnsafeAttribute
	));
	}

[mimoo]

do you think it'd make sense to merge this with the parse function above?

[katat]

This new parser function is for hint function without a body.
So I think once we support the native hints, we can just remove this function, while the native hint function can get parsed using parser function above without changes.