Proposal: Type bound functions

With the advent of functions with insert/update/delete, it's very tempting to be able to define functions on types themselves.
Schema
```edgeql
type Movie {
title: str;
multi actors: Actor;
multi reviews: Review;
getWithRating: function() {
select Movie {
title,
actors: {
name
},
rating := math::mean(.reviews.score)
}
}
}
```
And then:
```edgeql
with m := (select Movie limit 1), select m.withRating()
```

I know it's possible to do a similar behavior by passing a movie as the first parameter. But it's extremely convenient this way. And it would enable moving more of the business logic to the database with a minimal cognitive overhead.

If a schema has 40 types and each type has 2-3 functions:
- Currently, a developer would have to browse through 80-120 functions to find one to use.
- But with this, he wouldn't have to think about it. Plus it would be possible to get autocomplete in queries and client libraries ...

I don't have any stats, but I don't think people define functions all that much either in Postgres nor other databases. Maybe because ORMs don't expose that in a nice way, or maybe they'd rather implement stuff in the programming language.

I think this would be aligned with EdgeDB because it enables offloading more to the database while preserving programming language ergonomics.