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

[breaking-change]

Insert an `&` to fix
Regression test
auto merge of #18645 : nick29581/rust/coercions-1, r=alexcrichton

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)