differential-dataflow-v0.20.0

Other

• Update release notes for 0.20 (#680)
• Pull in probe changes (#677)
• Break Variable into (Variable, Collection) (#674)
• Update github workflows (#673)
• Set MSRV to 1.86 (#672)
• Differential corrected atop candidate timely 0.27 (#671)
• Test mdbook like a crate (#669)
• Migrate Join logic away from traits (#668)
• More VecCollection demotion (#667)
• Implement utility traits for Rc collections (#666)
• Deprioritize the Vec container (#664)

Substantial superficial but breaking changes due to the adoption of timely 0.27.
Primarily that the Stream contained by Collection is not Clone by default, and is now conventionally passed owned rather than by reference.
The necessary fix-ups are to convert to owned collections, and where needed to clone the collection, which requires that its container implement Clone.

Related, the Variable types is broken into a pair (Variable, Collection), where the latter is the usable collection the type would previously Deref to, and the former is the connection point to bind the variable's definition.
The SemigroupVariable was removed, as it is now redundant with Variable::new(), which does not require Negate (the main difference between the two variables).

Several steps were taken to "deprioritize" collections with vector containers.
Many traits defined only on vector collections were removed in favor of inherent methods.
Traits and methods that produce vector collections were (and continue to be) reconsidered to avoid routing users into that opinion.
All prior functionality is available, though in some cases it requires explicit indication of preferred container types (internally, only in "demo" cases).

The probe operator was updated to track timely changes that have it return a pair of handle and collection, rather than only a handle.
This aligns more with the "owned collection" idiom.