Add flatten() free function and fix flatten tests

[bluss]

This restores our travis to working order by not using .flatten() directly, since it is now incompatible with Rust nightly.

We introduce a free function flatten — similar to many other IntoIterator-enabled free functions we already have: https://docs.rs/itertools/0.7.6/itertools/#functions

The benefit of flatten as a free function is that we can support it from Rust 1.12.0 (version req right now) and for all the forseeable future. In a future version of itertools, this free function will be "upgraded" to use the std .flatten() iterator. The free function will remain while we will remove Itertools::flatten (unless std removes their version) in a future version.

Users that have compatibility problems with Itertools::flatten and its clash with Iterator::flatten, they can either use use itertools::Itertools; Itertools::flatten( expression ) or use itertools::flatten; flatten( expression ).