This moves all multi-function related code in the `functions` module into a new `multi_function` namespace. This is similar to how there is a `lazy_function` namespace. The main benefit of this is that many types names that were prefixed with `MF` (for "multi function") can be simplified. There is also a common shorthand for the `multi_function` namespace: `mf`. This is also similar to lazy-functions where the shortened namespace is called `lf`.