std: Use `do` blocks instead of `for` with .iter_bytes()
extra: Use `do` instead of `for` in extra::iter
std: Replace `for` with `do { .. }` expr where internal iterators are used
extra: Replace `for` with `do { .. }` expr where internal iterators are used
extra: Use external iterators in bitv implementation

Convert some internally used functions to use a external iterators.

Change all uses of remaining internal iterators to use `do` expr
std: Remove the internal iterator methods from trait Set

.intersection(), .union() etc methods in trait std::container::Set use
internal iters. Remove these methods from the trait.

I reported issue #8154 for the reinstatement of iterator-based set algebra
methods to the Set trait.

For bitv and treemap, that lack Iterator implementations of set
operations, preserve them as methods directly on the types themselves.

For HashSet, these methods are replaced by the present .union_iter()
etc.
std: Replace `for` with `do { .. }` expr in std::gc

Change all users of old-style for with internal iterators to using
`do`-loops.

The code in stackwalk.rs does not actually implement the
looping protocol (no break on return false).

The code in gc.rs does not use loop breaks, nor does any code using it.

We remove the capacity to break from the loops in std::gc and implement
the walks using `do { .. }` expressions.

No behavior change.
std: Change `Times` trait to use `do` instead of `for`

Change the former repetition::

    for 5.times { }

to::

    do 5.times { }

.times() cannot be broken with `break` or `return` anymore; for those
cases, use a numerical range loop instead.
rustc: add a lint for `for`, suggesting `foreach` or `do`.

This is just to aid the transistion to the new `for` loop, by
pointing at each location where the old one occurs.
migrate to foreach
auto merge of #8190 : thestinger/rust/for, r=thestinger