Disallow coercions from [T, ..n] to &[T]

r?

(I realise this needs a rebase, but I will probably have to chop it up in order to land and I'd like to get r+ first so I can do that quicker)
I'm under the impression that this change won't be necessary once #18486 lands. Since `[T, ..N] == [T, ..N]` works up to `N = 32`. cc @nikomatsakis