Unlock for i in 0..n loops! (by bumping rust-toolchain to nightly-2021-03-17)

[eddyb]

Most of the changes are uninteresting (and in their own commit), and were already part of #490.

What's new is @m-ou-se landed rust-lang/rust#83022 in nightly, decoupling mem::replace from mem::swap.
So while mem::swap still doesn't work for us (as rust-lang/rust#83019 wasn't approved), most things only use mem::replace, and that's in fact the case for Range's Iterator impl, which was the last thing that had been blocking for i in 0..n.

And after this nightly bump, we now have e.g. this loop in mouse-shader:

    for i in 0..3 {
        painter.fill(
            mouse_button(i),
            // ...
        );
    }

However, we/I haven't had a chance to test more complicated patterns of iterators, and it would be great if everyone started experimenting with all kinds of iterators (either base ones or combinators etc.), to get a feel for what works and what doesn't.