chore: move a couple of SharedMutableValues functions outside of impl

[asterite]

This was discussed a bit over slack, but here's some context.

In Rust this gives an error:

pub struct Foo<T> {
    x: T,
}

impl<T> Foo<T> {
    fn one(x: T) {
        Bar::two(x); // Cannot infer the value of const parameter U
    }
}

struct Bar<T, const U: usize> {
    x: T,
}

impl<T, const U: usize> Bar<T, U> {
    fn two(x: T) -> Foo<T> {
        Foo { x }
    }
}

fn main() {
    Foo::<i32>::one(1);
}

In the call Bar::two Rust needs to know what is the value of U. It can be solved by doing Bar::<T, 1234>::two(x) or using any numeric value... because the numeric value isn't used in that impl function... which probably means that it doesn't need to "belong" to Bar.

In this Noir PR gets merged that's what will happen. We have a similar case in this codebase where SharedMutableValues has two methods:

• unpack_value_change: doesn't refer to INITIAL_DELAY
• unpack_delay_change: doesn't refer to T

So, this PR moves those two methods outside of the impl, only specifying the generics that are needed.