Generics details 2: adapters, associated types, parameterized interfa…

…ces (#731)

This proposal goes into the details for these features of generics:

- adapters: for creating new types compatible with existing types but with different interface implementations
- associated types: allowing an interface implementation to specify some types to use in method signatures
- interface parameters: creating a family of interfaces, where types can implement more than one

This is a continuation of #553 . It has been summarized in these presentations:

- adapters: [1](https://docs.google.com/presentation/d/1bg6q0Q9Sk4YpRbNA3D3H34xYtaEO8ScAUNUZK2UTi80/edit?resourcekey=0-6-Y6e1mfRUmHg-Zk65Gc5A#slide=id.gcf40df1c7b_0_37) and [2](https://docs.google.com/presentation/d/17KG0TeJ4OChMRdLJPS8TE_K6SoL4lFy1FUGr2CDzX-A/edit?resourcekey=0-kLnZqd5NrbGSwmbunTyB-A#slide=id.g7a37009490_0_0)
- [associated types and interface parameters](https://docs.google.com/presentation/d/19hPpUjxQ0H1lUSLy5QjS2910Cpc7UdNKpF580fFsCGw/edit?resourcekey=0-ky9XGRC1I8X0Ffw6eqh7WQ#slide=id.p)

Co-authored-by: Wolff Dobson <wolffg@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>