Replace `mpmc_large` feature using `LenType`.

[reitermarkus]

It should be possible to replace the mpmc_large feature by using LenType, or possibly an AtomicLenType super trait.

[bmgxyz]

Is this issue wontfix in light of the deprecation of mpmc?

[sgued]

No, I think we'd still review and merge a PR for that, but it would be a breaking change so it would take some time before being merged (essentially not before we decide to do a major release, and the main reason we would want to do a major release is for a change in the MPMC algorithm that would be truly lock-free).