modules: Add a modules directory for mcl code

Details in the README file.

test/test-modules.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# check that our modules still build, even if we don't run them here
+
+# shellcheck disable=SC1091
+. test/util.sh
+
+echo running "$0"
+
+ROOT=$(dirname "${BASH_SOURCE}")/..
+cd "${ROOT}"
+
+failures=''
+
+# Test modules/ directory to see if the .mcl files compile correctly.
+
+find_mcl_modules() {
+	git ls-files | grep '\.mcl$' | grep '^modules/' | grep -v 'examples/lang/'
+}
+
+# TODO: It might be better to only test from the root module entrypoint.
+for file in $(find_mcl_modules); do
+	#echo "mcl: $file"
+	run-test ./mgmt run --tmp-prefix lang --only-unify "$file" &> /dev/null || fail_test "could not compile: $file"
+done
+
+if [[ -n "$failures" ]]; then
+	echo 'FAIL'
+	echo "The following tests (in: ${linkto}) have failed:"
+	echo -e "$failures"
+	echo
+	exit 1
+fi
+echo 'PASS'