[Pre-RFC] Fixed-capacity view of Vec

[scottmcm]

@scottmcm

Note that we have something very similar to this: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.repeat , which is even more efficient than cycle-take by using doubling to copy larger-and-larger chunks:

https://github.com/rust-lang/rust/blob/8a2dec6e583bc6425a91b277bdc6c602088845f1/src/liballoc/slice.rs#L409-L463

So, who else read that source code snippit carefully enough to spot the unsoundness?

[stable] std: Check for overflow in `str::repeat`