Include a basic Rust wrapper for Equi-X and HashX

The idea behind this is that we may want to start exporting more pieces of c-tor as Rust crates so that Arti can perform cross compatibility and comparison testing using Rust tooling.

This turns the 'tor' repo into a Rust workspace, and adds one crate to start with: "tor-c-equix", rooted in src/ext/equix. This actually includes both EquiX and HashX, since there's less overall duplication if we package these together instead of packaging HashX separately as well.

This patch adds a basic safe Rust interface, but doesn't expose any additional internals for testing purposes.

No changes to the C code here or the normal Tor build system.

Signed-off-by: Micah Elizabeth Scott beth@torproject.org