Deprecate Vec::get and Vec::get_mut

[nrc]

In favour of indexing (#15652). The plan as discussed in #15652 is to deprecate get and get_mut now, change all uses in the compiler to indexing, then remove at some point in the future. At some other point >= the future we reinstate get/get_mut as returning Option<T>, but that stage is to be confirmed later when we have a wider discussion about ergonomics.

[nrc]

Oh, this needs to wait for a snapshot once #15652 lands.

[nrc]

I was wrong, this didn't need a snapshort and the deprecation of get at least happened in #15652.