Support im 0.15

[kaspar030]

im-rs: "Blazing fast immutable collection datatypes for Rust."

[svix-jplatte]

Note that im has been unmaintained for a while, there is a fork called imbl that's more active.

[kaspar030]

Note that im has been unmaintained for a while, there is a fork called imbl that's more active.

Looking at the crates.io downloads, im is being downloaded 15x-20x more than imbl.
Also, imbl only implements a subset of ims features (notably, the non-Arc but Rc variant is missing).

As the implementation here is pretty small, is there another reason for not adding support?