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>

doc/HACKING/Rust.md

@@ -0,0 +1,43 @@
+# Rust support in C Tor
+
+The [Arti project](https://gitlab.torproject.org/tpo/core/arti) is the team's
+ongoing effort to write a pure-Rust implementation of Tor.
+
+Arti is not yet feature complete but it's in active development. That's where
+you want to go if you're interested in Tor and Rust together.
+
+This document describes something with niche interest: the C implementation of
+Tor can expose Rust crates which are used for internal testing, benchmarking,
+comparison, fuzzing, and so on. This could be useful for comparing the C
+implementation against new Rust implementations, or for simply using Rust
+tooling for writing tests against C.
+
+## Crates
+
+Right now we are only using this mechanism for one crate:
+
+- `tor-c-equix` -- Wraps the `src/ext/equix` module,
+  containing Equi-X and HashX algorithms.
+
+## Stability
+
+This is not a stable API and we have no plans to develop a stable Rust interface
+to the C implementation of Tor.
+
+## Files
+
+We use only a few of the standard Rust file types in order to build our
+wrapper crates. Here's a summary:
+
+- `Cargo.toml` in the repository root defines a Cargo *workspace*. It will
+  list all subdirectories that contain crates with their own `Cargo.toml`.
+- A per-crate `Cargo.toml` defines metadata and dependencies. These crates
+  should all be marked `publish = false`.
+- `build.rs` implements a simple build system that does not interact with
+  autotools. It uses the `cc` and `bindgen` crates to get from `.c`/`.h`
+  files to a static library and matching auto-generated bindings. Prefer to
+  include bindgen wrapper headers inline within `build.rs` instead of adding
+  `.h` files that are only used by the Rust bindings.
+- `lib.rs` publishes the low-level `ffi` interface produced with `cc` and
+  `bindgen`. This is also where we can add any wrappers or additions we want
+  for making the Rust API more convenient.