Add hypothetical support for ranges with only an upper bound

Note that this doesn't add the surface syntax.
Remove ExprSlice by hacking the compiler

[breaking-change]

The `mut` in slices is now redundant. Mutability is 'inferred' from position. This means that if mutability is only obvious from the type, you will need to use explicit calls to the slicing methods.
Fallout from mut slices
Rebasing and reviewer changes
auto merge of #20160 : nick29581/rust/ranges2, r=nikomatsakis

The first six commits are from an earlier PR (#19858) and have already been reviewed. This PR makes an awful hack in the compiler to accommodate slices both natively and in the index a range form. After a snapshot we can hopefully add the new Index impls and then we can remove these awful hacks.

r? @nikomatsakis (or anyone who knows the compiler, really)