`Report::frames_mut` allows aliased mutable references

Affected versions of this crate return an iterator from
`Report::frames_mut` whose `&mut Frame` items have lifetimes independent
of the iterator, so all yielded references can be held at the same time.
A yielded frame's sources are also yielded by the iterator and reachable
through the parent frame via `Frame::sources_mut`, allowing safe code to
obtain two live mutable references to the same frame. This is undefined
behavior and can be used to cause a segmentation fault (see reproducer in
the linked issue). Versions 0.1.0 and 0.1.1 are affected through
`Frame::source_mut` instead of `Frame::sources_mut`.

The flaw is fixed in 0.8.0 by replacing the iterator with internal
iteration: `Report::frames_mut` now takes a visitor closure, so mutable
access to a frame is scoped and cannot overlap with access to its
sources.