generator: Replace Extends{Root} with StructExtends<Root> generic

Instead of emitting a new `trait` for every `Root` struct that is being
implemented by one or more "child" structs (those that have `Root` in
their `structextends`), create one trait that takes the root struct as a
generic parameter, and implement that directly instead.

This not only saves on having to define the `trait` for every
`Root` struct but also paves the way towards providing default trait
implementations for any pair of root and child struct, such as the
`p_next` builder methods.

Changelog.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     - `get_pipeline_executable_statistics()`.
   The expected length of this array can be queried with the respective `*_len()` variant of these functions.
 - `push_next()` has been renamed to `extend()` and marked as `unsafe`. Users are encouraged to call `push()` for singular structs instead. (#909)
+- All `Extends{Root}` traits have been replaced with a single `Extends<Root>` trait using generics. (#971)
 
 ### Removed