* fix typing support for python 3.8
* made `readenv.redenv.load` "private" (`readenv.readenv._load`)
* rename `readenv.readenv.loads` to `readenv.readenv.load`
* export `readenv.readenv.load` as `readenv.load`
* added `readenv.env.get`, `readenv.env.set`, and `readenv.env.setdefault`,
and exported to `readenv`
* added type converters