Stabilize user-defined functions

User-defined functions were added in #3247 with the following syntax:

```just
foo(a, b) := a + b
```

They are currently unstable, since I'm not sure about the syntax. The alternative that I'm considering is:

```just
fn foo(a, b) := a + b
```

I personally think the syntax without `fn` looks better, but with `fn` allows grepping for function definitions with `^fn`, however I'm not sure if that's a huge advantage.

Feedback on the syntax is most appreciated, in either direction. I'd like to stabilize user-defined functions soon, since it's only the syntax which is holding them back.