[rust] Add .cargo/config

This allows us to have a unified source for vendored crates as well
as a unified target directory for all fargo build.

This CL also adds Cargo.toml files to the .gitignore since we no
longer check them in as part of the build system.

Change-Id: I1d19404f6b1760c2a5931fd9984e6093e195816a