te feature introduced

[swasilyev]

May be temporary, to advance pasta-tree that is SW. Fuck @drskalman. Will break CI where "--no-default-features"

[davxy]

This breaks more than CI.

• Feature on -> SW gated out;
• Feature of -> crate doesn't build

Other concerns:

• Mutually-exclusive Cargo feature: Features are meant to be additive/unifiable. Any crate pulling w3f-ring-proof with defaults forces twisted_edwards tree-wide; SW consumers can't opt out without --no-default-features. Previously both impls coexisted in one build. This removes that
• SW tests silently dropped. Under a workspace cargo test feature unification builds w3f-plonk-common with twisted_edwards on, so sw-cond_add (and test) is cfg'd out and stops running with no warning.

Both forms must remain exposed in the same build