Rust-itertools

- Write `iproduct!()` as a single expression; this allows temporary values in its arguments.

- Add `.fold_options()`
- Require Rust 1.1 or later

- Update `.dropping()` to take advantage of `.nth()`

- `.merge()`, `.unique()` and `.dedup()` now perform better due to not using function pointers
- Add free functions `enumerate()` and `rev()`
- Breaking changes:
- Return types of `.merge()` and `.merge_by()` renamed and changed
- Method `Merge::new` removed
- `.merge_by()` now takes a closure that returns bool.
- Return type of `.dedup()` changed
- Return type of `.mend_slices()` changed
- Return type of `.unique()` changed
- Removed function `times()`, struct `Times`: use a range instead
- Removed deprecated macro `icompr!()`
- Removed deprecated `FnMap` and method `.fn_map()`: use `.map_fn()`
- `.interleave_shortest()` is no longer guaranteed to act like fused

- Rename `.sort_by()` to `.sorted_by()`. Old name is deprecated.
- Fix well-formedness warnings from RFC 1214, no user visible impact

- Improve performance of `.merge()`'s ordering function slightly