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.)

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