Move Cargo.toml to top level

Instead of requiring cargo to be run in the sim directory, create a
top-level "workspace" Cargo file, that references the others.  The main
result of this is that cargo can now be run in the top of the workspace,
and the 'target' directory will be placed there, rather than in the sim
directory.

This is primarily of benefit to tools such as RLS, Rust-analyzer and
various IDEs that expect a crate at the top level of the tree.

Signed-off-by: David Brown <david.brown@linaro.org>