Consider combining implementations of `walk*_vectors_or_scalars` with fold instructions

[mmichel11]

          I *think* it should be possible to combine `walk*_vectors_or_scalars` together with some complicated fold instructions, lambdas, tuples, and std::apply.  

Take a look at the first answer of https://stackoverflow.com/questions/7230621/how-can-i-iterate-over-a-packed-variadic-template-argument-list. I think you should do something similar, chaining together instructions by returning tuples and then with std::apply.

Here is an example I was playing with.
https://godbolt.org/z/vc8dK4ed6

In the end, I'm not sure if (1) its actually possible and (2) its worth the complexity to consolidate these structs, but its worth considering...

Originally posted by @danhoeflinger in #1976 (comment)

The current implementation of walk*_vectors_or_scalars introduced in #1976 manually implements each path separately. This is due to the difficulty of combining the separate vector paths in a single unified implementation. However, there is a common structure shared among all three functors and @danhoeflinger's recommendation looks implementable.

When exploring combining the implementations, we should consider readability / complexity of the fold instruction approach when determining if we should adopt it. Additionally, the walk1_vector_or_scalar implementation should avoid explicit copying (as currently implemented), so the user's unary functor in std::for_each can still directly modify the reference.