Add iterator to List, Set and Map

[davidrusu]

Adds List::iter(&self)

I'll likely be wanting iterators for Set and Map soon as well, I'll send PR's as needs come up

[davidrusu]

Updated to use impl Trait which just landed in Rust 1.26 stable, see: https://blog.rust-lang.org/2018/05/10/Rust-1.26.html

[davidrusu]

Since the impl Trait syntax makes things so convenient, I went ahead and added iter to map and set

[flosse]

@alex-shapiro do you mind to merge it?

[davidrusu]

@flosse I've been experimenting with the design of a CRDT library over at: https://github.com/rust-crdt/rust-crdt You may be interested to poke around (readme and published docs haven't been updated so you'll have to read the code to see the latest design, I'll get around to updating it soon).

[ngortheone]

@davidrusu it looks like @alex-shapiro has abandoned this repo :(