RangeArgument not exported to std::collections

[Pscheidl]

Trait RangeArgument (currently new and unstable in Rust 1.3) is not exported into standard library.
https://doc.rust-lang.org/stable/collections/range/trait.RangeArgument.html

[alexcrichton]

This is currently intentional because the enum is unstable (and nightly users can get it through libcollections), co closing as working as intended. Thanks for the report though!

Also note that soon this enum may not be necessary