rust: Use Cargo to build the Rust portion

Pros: Ability to use Rust libraries from crates.io
Problems: Need to lock library versions and document their reproducible building, either via Cargo.lock or vendoring.

cargo.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -e
+
+export CARGO_TARGET_DIR=`pwd`
+if [ ! -z ${2} ]; then
+    OUT_PATH=`realpath "${2}"`
+fi
+
+cd $1
+cargo $3 -p rs
+
+if [ ! -z ${OUT_PATH} ]; then
+    cp "${CARGO_TARGET_DIR}"/debug/librs.a "${OUT_PATH}"
+fi