Add iter! macro

[malbarbo]

A temporary solution for the missing IntoIterator implementation for [T; N] (rust-lang/rust#25725). While I was finishing this PR another issue was opened rust-lang/rust#39257.

When IntoIterator get implemented for [T; N] an user can port their code removing the iter! macro invocations.

My main use case for this is to use the free functions in itertools, like max, with an arbitrary number of elements. The benchmarks show no difference for maxN(a, b, ...) and max(iter!(a, b, ...)).

This introduces a dependency on nodrop.

[bluss]

Hi, this is of course useful but it's not necessarily for itertools, it's better as its own crate, especially since it involves a series of hacks (It's unfortunate that Rust does not yet support it better, simply!). Previous existing ones solutions were literator and arrayvec, I guess.

[bluss]

Closing with the motivation in preceding comment.