Work around swap_bytes on WebAssembly

[CryZe]

Unfortunately neither u128 nor swap_bytes are supported directly by WebAssembly. So both implementations of folded_multiply are very slow.

I think an algorithm that takes both u64 values, turns them into a v128 vector and then does a bunch of swizzling and vector multiplications and co. probably would be the much faster solution. Here a Godbolt link with a little sketch:

https://rust.godbolt.org/z/jGGhYjGs8

I don't have enough knowledge about how to verify the quality, so I decided to not directly open a PR and instead first discuss the feasibility.

[tkaitchuck]

Do you have a good way to benchmark this? If so open a PR for that, and I'll play with it to make sure that quality is there.