Add rust::Vec accessors

[rinon]

Adds operator[], at(), front(), and back() to rust::Vec.

This is the read-only accessor side of #149, not sure if we want mutable accessors.

[rinon]

These accessors have the same (un)safety as the corresponding accessor for std::vector. Not sure if we want that, but it's a starting point.

I added no_exceptions checks for at(), which throws in std. AFAICT this is the first thing that should throw. Do you want to support exceptions?

[dtolnay]

Thanks! This is great.

I think matching the (un)safety of the equivalently named std::vector APIs is the right call.

For exceptions, in #92 we had landed on the approach in #101. I am going to make a follow up PR to unify how Vec::at does exceptions with what String's and Str's constructors are doing.

[dtolnay]

not sure if we want mutable accessors.

Yes, I think we do want these!