remove feature opt-ins that are no longer needed

[dwrensha]

On rustc 1.17.0-nightly (cab4bff3d 2017-03-21) I see these warnings:

warning: this feature has been stable since 1.17.0. Attribute no longer needed
 --> src/lib.rs:5:5
  |
5 |     pub_restricted,
  |     ^^^^^^^^^^^^^^
  |
  = note: #[warn(stable_features)] on by default

warning: this feature has been stable since 1.17.0. Attribute no longer needed
 --> src/lib.rs:2:5
  |
2 |     btree_range,
  |     ^^^^^^^^^^^
  |
  = note: #[warn(stable_features)] on by default

warning: unused or unknown feature
 --> src/lib.rs:3:5
  |
3 |     collections,
  |     ^^^^^^^^^^^
  |
  = note: #[warn(unused_features)] on by default