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 Cargo workspace, and adds one crate to
start with: "tor-c-equix", rooted in src/ext/equix. This actually
includes both Equi-X itself and HashX, since there's less overall
duplication if we package these together instead of packaging HashX
separately.

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>