1. Adds an empty .rustfmt.toml so formatting for this project will be respected in workspaces with different formatting.
2. Adds unstack, which is the inverse of stack.

Note: I've never before made such liberal use of the Rust type system, so I basically did a lot of pattern-matching with stack to figure out what to do.
You'll probably want to review it carefully.
(The tests do pass, though.)

Note: I'm not confident in my ability to write correct CUDA kernels using your library, so I only implemented the CPU backend.