iterator: Add `IteratorUtil::flat_map_` method

flat_map_ produces an iterator that maps each element to an iterator,
and yields the elements of the produced iterators.

This is the monadic bind :: M a -> (a -> M b) -> M b  for iterators.

Named just like the vec method, but with a trailing underline until the
method resolution bug is resolved.
driver: perform stripping before and after macro expansion.

This allows macros to both be conditionally defined, and expand
to items with #[cfg]'s.
auto merge of #7297 : huonw/rust/strip-expand-strip, r=cmr

This allows macros to both be conditionally defined, and expand
to items with #[cfg]'s.

This seems to have a performance improvement, e.g. for `std`:

```
# Before 
time: 1.660 s   expansion
time: 0.125 s   configuration
# After
time: 0.080 s   configuration 1
time: 1.127 s   expansion
time: 0.132 s   configuration 2
```

And for `extra`:

```
# Before
time: 0.593 s   expansion
time: 0.062 s   configuration
# After
time: 0.047 s   configuration 1
time: 0.147 s   expansion
time: 0.058 s   configuration 2
```

(This seems a little peculiar, but it is possibly because the expansion AST traversal is very slow, so removing as much as possible as early as possible has big benefits.)
auto merge of #7345 : blake2-ppc/rust/iterator-flat-map, r=thestinger

flat_map_ produces an iterator that maps each element to an iterator,
and yields the elements of the produced iterators.

This is the monadic bind :: M a -> (a -> M b) -> M b  for iterators.

Named just like the vec method, but with a trailing underline until the
method resolution bug is resolved.

We discussed the name chain_map, but I decided to go with flat_map_ for consistency with vec.

Since it.map(f).flatten()  would be the same as it.flat_map(f), we could choose
to just implement a flatten method instead. Either way the possibilities are the same but flat_map is more convenient.