An associated type per implementation

Decided

D-0029, settled by the language’s author: part of spec §18. Older text cites it as 18.29.

Each implementation chooses its own. The associated type belongs to the impl, not the type, so impl Mul<Vec4> for Mat4 answers Output = Vec4 and impl Mul<f32> for Mat4 answers Output = Mat4. Falls out of 18.31; Mul<Rhs, Output> with a defaulted second parameter was the alternative, and would have needed a rule forbidding two outputs for one right-hand side (§8.5, §11.2)