Upgrade to latest Rust

This includes two changes that landed recently:
- Replace ~T with Box<T>, ~value with box value
- local_data API changes

Attention:
This includes some changes in 32-bit-only code that I don't have the hardware to test. I've been careful not to make any mistakes there, but I can't guarantee that I haven't broken 32-bit builds. Does Travis build on 32-bit platforms?