Add a RUST-TODO.md to track progress of rust impl
Compile rust in to bin/ directory.

Remove the old symlinks from bin/. Add bin/redo.do to build the rust
implementation, bin/sh.do to determine the shell (replacing
redo-sh.dir.do), and bin/test.do to run the embedded unit tests (not
that there are any yet).

Modify _all.do and test.do to include the new targets.
TODO: Directory printing
Expand testing slightly

Include the smoke test, a basic unit test, and run the tests under t/
using bin/redo. Also, detect implementations that don't build properly
and fail in those cases too.