Do The SIMD Shuffle

A "shuffle", in SIMD terms, takes a SIMD vector (or possibly two vectors) and a pattern of source lane indexes (usually as an immediate), and then produces a new SIMD vector where the output is the source lane values in the pattern given.

Example (pseudo-code):

let a = simd(5, 6, 7, 8);
assert_eq!(shuffle(a, [2, 2, 1, 0]), simd(7, 7, 6, 5));

Shuffles are very important for particular SIMD tasks, but the requirement that the input be a compile time constant complicates the API:

• Rust doesn't have a stable way for users to express this, so they never expect it.
• Rather than just having a const-required arg (which totally works) a lot of people have said "you should use const generics for that!" (which actually doesn't work as well on current rust).

Still, min_const_generics is aimed to be stable by the end of the year and most likely that'll be enough to do shuffle basics on stable.

The issue provides a conceptual description and Rust pseudo-code for SIMD shuffles, but names no files, tests, entry points, or concrete acceptance criteria. Start by clarifying the stable API and const-generic design with the maintainers; done would require an agreed implementation scope and corresponding validation.