Remove 'cl dependency

This only required two things: replacing the calls to `first' and
`second' in `ht-from-plist' with `car' and `cadr' respectively.
Add the ability to pass an TEST argument to `hn-create'

This is mainly useful in case you need to create an hash table with
the test `equalp'.

The default value of TEST is still `equal'.
Add `ht-to-alist` and `ht-to-plist`

Add both functions to README.md