Add generic associated types (GAT)

This is necessary to support mapper functions, such as `map`, `filter` and `filter_map`, both on containers and iterators.

A design [similar to Rust](https://doc.rust-lang.org/stable/reference/items/associated-items.html) is probably the way to go.

The issue names no repository files, tests, or entry points. Start by reviewing the linked Rust associated-items design and the language's type-system implementation, then determine how generic associated types should support containers and iterators. Done means a settled design and working support for map, filter, and filter_map.

Written by the indexing model from the issue text.