Replace NoDrop with equivalent MaybeUnit-like union

[bluss]

Stop using the nodrop crate in arrayvec.

We need (to replace nodrop with something) sound inclusion of uninitialized inline array values and soundness vs the Option Some/None issue.

[bluss]

This was implemented conditionally in latest 0.4.x and master in PR #114, but the issue not solved until that's available on stable.

[bluss]

Implemented conditionally (Rust 1.36 or later) in #127

[bluss]

Fixed by #129