More robust iteration over Vectors (#27079)
* More robust iteration over Vectors
Currently, if a vector is resized in the midst of iteration, then `done` might "miss" the end of iteration. This trivially changes the definition to catch such a case. I am not sure what guarantees we make about mutating iterables during iteration, but this seems simple and easy to support. Note, though, that it is somewhat tricky: until #13866 we used `i > length(a)`, but that foils vectorization due to the `typemax` case. This definition seems to get the best of both worlds. For a definition like `f` below, this new definition just requires one extra `add i64` operation in the preamble (before the loop). Everything else is identical to master.
```julia
function f(A)
r = 0
@inbounds for x in A
r += x
end
r
end
```
