This adds a couple of configure commands to control whether we're
requiring all dependencies to be available locally (default) or not
(--enable-cargo-online-mode). When building from a tarball, we require
the RUST_DEPENDENCIES variable to point to the local repository of
crates. This also adds src/ext/rust as a git submodule that contains
such a local repository for easy setup.
This gives an indication in the log that Tor was built with Rust
support, as well as laying some groundwork for further string-returning
APIs to be converted to Rust
