The meaning of rotate_left/right

[nixberg]

Switching a project from packed_simd_2 to simd_core, I noticed that rotate_left and rotate_right have become horizontal operations, borrowing from std::slice.

I believe this to be inconsistent and surprising

• since I consider SIMD types arithmetic types first,
• since rotate_left is (conceptually) just (x << n) | (x >> (BITS - n)) and these constituent operations remain vertical,
• since rotate_left/right already exist in std as (trivially) vertical operations.

Also, there is precedent for horizontal_ operations but not for explicitly vertical ones.

Therefore, whether or not simd_core ends up providing vertical rotation, I propose renaming the methods in question to something like horizontal_rotate_left.

[calebzulawski]

Relevant discussion: #14 (comment)