Update to latest Rust
Derive Copy wherever sensible

Copy is now opt-in, see [1]. To keep the current library functionality it is
derived for all sensibly copyable types.

[1]: https://github.com/rust-lang/rust/commit/096a28607fb80c91e6e2ca64d9ef44c4e550e96c
Merge pull request #53 from aepsil0n/update_nightlies

Update to latest Rust