Deprecate Vec::get and Vec::get_mut #15672
Description
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.