Add enum discriminants (#337)

[obi1kenobi]

• Remove nightly from rustdoc v28 test matrix, since that's now v29. (Remove nightly from rustdoc v28 test matrix, since that's now v29. #332)

• Rust 1.78 is no longer beta. (Rust 1.78 is no longer beta. #334)

• Add enum discriminants

• discriminant clone to lifetime annotation

• complicate test examples

• improved discriminant docs

• hedge bets against placeholder representation

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• finally got implicit discriminants working

• remove sync structs

• move to Cow<'a, str> for discriminants

• Various clean-ups, still fighting closure bounds

sigh

error[E0521]: borrowed data escapes outside of closure
--> src/adapter/edges.rs:304:28
|
260 | pub(super) fn resolve_variant_edge<'a, V: AsVertex<Vertex<'a>> + 'a>(
| -- lifetime 'a defined here
...
302 | "discriminant" => resolve_neighbors_with(contexts, move |vertex: &'_ Vertex<'a>| {
| ------ - let's call the lifetime of this reference '1
| |
| vertex is a reference that is only valid in the closure body
303 | let origin = vertex.origin;
304 | let enum_var = vertex
| ____________________________^
305 | | .as_variant()
| | ^
| | |
| |_____________________________vertex escapes the closure body here
| argument requires that '1 must outlive 'a

• Actually clone Cow; same error as before, though...

• Got the sucker.

• Wrap up

• Better dcocs for discriminant

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com

• Finish up docs, improve tests

• Add name back.

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Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com