Add an optional C-implementation for configmix.config.unquote and configmix.config.pathstr2path. This is currently for Python 3.5+. It is tested with Python 3.7 and Python3.8 (FreeBSD 12.2 amd64, LLVM 10.0.1). A build for the stable API ("abi3") fails because PyUnicode_New() is currently not in the stable API. Also includes are extended tests for unquote() and pathstr2path().

{"key1": "in the root namespace",
 "key2": "in the root namespace -- too",
 "tree1": {
     "key3": 32,
     "tree2": {
	 "key4": "get this as `tree1.tree2.key4'",
	 "key5": true,
	 "key6": "off"
     }
 }
}