Add recipe for RustPython

I'm a contributor to RustPython; we recently got support for Redox running on master and I figured I'd open a merge request here. The branch the recipe is based on is redox-release, which I manage and I'll make sure compiles for Redox before pushing to Github.

recipes/rustpython/recipe.sh

+GIT=https://github.com/RustPython/RustPython
+BRANCH=redox-release
+CARGOFLAGS=--no-default-features
+export BUILDTIME_RUSTPYTHONPATH=/lib/rustpython/
+
+function recipe_stage() {
+  dest="$(realpath "$1")"
+  mkdir -pv "$dest/lib/"
+  cp -r Lib "$dest/lib/rustpython"
+}