auto merge of #20163 : bfops/rust/master, r=Gankro

TODOs:
  - ~~Entry is still `<'a, K, V>` instead of `<'a, O, V>`~~
  - ~~BTreeMap is still outstanding~~.
  - ~~Transform appropriate things into `.entry(...).get().or_else(|e| ...)`~~

Things that make me frowny face:
  - I'm not happy about the fact that this `clone`s the key even when it's already owned.
  - With small keys (e.g. `int`s), taking a reference seems wasteful.

r? @Gankro
cc: @cgaebel

diff --cc src/librustc/session/config.rs
Simple merge