Due to linker issues (#25386) when testing Rust code which calls C,
all tests which touch FFI code should now be feature-gated behind the
"test-c-from-rust" flag. To run this test code, cargo must be called
with `cargo test --features="test-c-from-rust"`.
* FIXES#26398: https://bugs.torproject.org/26398
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
