lane -> element for core::simd::Simd

A while ago we began saying T, N instead of T, LANES in reference to Simd.
At some point that leaked in to us checking in code with const N: usize.
After a while, we had a discussion and agreed that "lanes", while common,
is unnecessary jargon for Rust learners who aren't familiar with SIMD, and
is fully interchangeable with terms for arrays like element and index.

But we never acted on that. Let's update the main type's docs, at least.
The example tweaks also enable removing a slated-for-removal nightly fn.

Author: workingjubilee