Replace implementation of .join() with a version using fmt::Display
Replace implementation of .join() with a version using fmt::Display

Mark .to_string_join() deprecated.

Since all implementors of Str, should impl fmt::Display the same way,
this should be mostly backwards compatible. On breakage you may have to
insert a mapping from your `T: Str` value to `&str`.

This version of `.join()` is more general, performs the same amount of
work for strings, and is efficient enough for all use cases.

flo-l/rust-itertools,imp/rust-itertools,Wilfred/rust-itertools,DanielKeep/rust-itertools,stepancheg/rust-itertools,tempbottle/rust-itertools,rust-itertools/itertools