Rename the 'cretonne' crate to 'cretonne-codegen'.

This fixes the next part of #287.

lib/codegen/meta/check.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -euo pipefail
+cd $(dirname "$0")
+
+function runif() {
+    if command -v "$1" > /dev/null; then
+        echo "   === $1 ==="
+        "$@"
+    else
+        echo "$1 not found"
+    fi
+}
+
+# Style linting.
+runif flake8 .
+
+# Type checking.
+runif mypy --py2 build.py
+
+# Python unit tests.
+runif python -m unittest discover
+
+# Then run the unit tests again with Python 3.
+# We get deprecation warnings about assertRaisesRegexp which was renamed in
+# Python 3, but there doesn't seem to be an easy workaround.
+runif python3 -Wignore:Deprecation -m unittest discover