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

[MarijnS95]

Works towards #879, yet without swapping the generic and the struct to implement it for.

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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.

[MarijnS95]

It looks like this PR comes with a ±150ms build speedup:

master (92a0ee6)

$ hyperfine -p 'cargo clean -p ash' 'cargo b -p ash'
Benchmark 1: cargo b -p ash
  Time (mean ± σ):      7.964 s ±  0.109 s    [User: 7.768 s, System: 0.902 s]
  Range (min … max):    7.797 s …  8.133 s    10 runs

This PR (1cf5047)

$ hyperfine -p 'cargo clean -p ash' 'cargo b -p ash'
Benchmark 1: cargo b -p ash
  Time (mean ± σ):      7.817 s ±  0.092 s    [User: 7.669 s, System: 0.845 s]
  Range (min … max):    7.707 s …  8.008 s    10 runs

[Ralith]

Awesome! Glad we finally stumbled on something that helps there, and great that it's a readability/concision win too.

[MarijnS95]

Sneak-preview: moving the functions to AnyTaggedStructure is going to give us another ±90ms :)

[MarijnS95]

Performance optimization is now less significant, ±105ms, perhaps because of the expanded lifetimes?

(Retested master too and its performance measurement was within margin of error on my machine)

This PR (4305ec7):

❯ hyperfine -p 'cargo clean -p ash' 'cargo b -p ash'
Benchmark 1: cargo b -p ash
  Time (mean ± σ):      7.858 s ±  0.103 s    [User: 7.702 s, System: 0.876 s]
  Range (min … max):    7.745 s …  8.026 s    10 runs

[Ralith]

We're done here, right? I'm happy to discuss changing the lifetime semantics to try to recapture some of that buildtime if you want, but it's a net win regardless.