rust: Prepare for more versions part 1:
 - Factor out some things that REALLY shouldn't change between versions
rust: Prepare for more versions part 2:
 - Default is now an alias for 0.11
rust: Add hashes for i686-darwin
rust: Add an alternative package for the master tip

based off of pull request #3084
rust: Refer to 0.11.nix directly in all-packages
rather than use default.nix indirection
rustc: rename rust to rustc, but keep rust attr name for backwards compat
Merge pull request #3412 from Ericson2314/rust-master

Add alternative rustc package for master tip