itertools breaking changes

`RepeatCall::new` to `repeat_call()`

`Zip::new` to `multizip`

`PutBack::new`, `PutBackN::new` to `put_back`, `put_back_n`

[Release announcement](https://bluss.github.io/rust/2016/09/26/itertools-0.5.0/)

`combinations` is now `tuple_combinations`

`combinations_n` to `combinations`

`group_by_lazy`, `chunks_lazy` to `group_by`, `chunks`

`MultiPeek::new`, `.multipeek()` to `multipeek()`

`.into_rc()` to `rciter`

`Partition` enum is now `Either`

Module reorganization:

All iterator structs are under `itertools::structs` but also reexported to the top level, for backwards compatibility

`PutBack::with_value` is now a builder setter, not a constructor

`ZipSlices`, use `.zip()` instead

`.enumerate_from()`, `ZipTrusted`, due to being unstable

`.mend_slices()`, moved to crate `odds`

Stride, StrideMut, moved to crate `odds`

`linspace()`, moved to crate `itertools-num`

`.sort_by()`, use `.sorted_by()`

`.is_empty_hint()`, use `.size_hint()`

All free functions are reexported at the root, `itertools::free` will be removed in the next version

`.merge()`, `.unique()` and `.dedup()` now perform better due to not using function pointers

Add free functions `enumerate()` and `rev()`

Breaking changes:

`.interleave_shortest()` is no longer guaranteed to act like fused

`.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 deprecated macro `icompr!()`

Method `Merge::new` removed

Removed deprecated `FnMap` and method `.fn_map()`: use `.map_fn()`

Removed function `times()`, struct `Times`: use a range instead

Return types of `.merge()` and `.merge_by()` renamed and changed