auto merge of #9108 : blake2-ppc/rust/hazards-on-overflow, r=alexcrichton

Fix uint overflow bugs in std::{at_vec, vec, str}

Closes #8742

Fix issue #8742, which summarized is: unsafe code in vec and str did assume
that a reservation for `X + Y` elements always succeeded, and didn't overflow.

Introduce the method `Vec::reserve_additional(n)` to make it easy to check for
overflow in `Vec::push` and `Vec::push_all`.

In std::str, simplify and remove a lot of the unsafe code and use `push_str`
instead. With improvements to `.push_str` and the new function
`vec::bytes::push_bytes`, it looks like this change has either no or positive
impact on performance.

I believe there are many places still where `v.reserve(A + B)` still can overflow.
This by itself is not an issue unless followed by (unsafe) code that steps aside
boundary checks.

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