Remove the returns library dependency

[chuckwondo]

I originally introduced the returns library in order to experiment with writing more Functional Programming oriented Python.

While it was a worthwhile experiment, I'm in favor of removing it for at least the following reasons:

1. Most importantly, it seems to swallow tracebacks, making it extremely difficult (if not impossible) to debug errors
2. The types and type hints are quite complex (perhaps convoluted), making it extremely difficult to decipher typing errors, and thus making it difficult to know how to correct such errors (or even to know when to ignore them)
3. Various parts of the API are perhaps a bit convoluted, making our code hard to read. In part, this is likely due to the introduction of FP-style of code, which can be a bit cryptic to those unfamiliar with various FP concepts, so this is not completely unexpected. However, the library itself seems to only compound the difficulty in deciphering the code.

At another point, I would like to consider what appear to be a cleaner, simpler, more approachable, and more robust FP libraries (pfun or pymonad), as I believe exploring FP concepts again/further is still worthwhile, particularly in the context of a smaller codebase, such as this one. However, for now, I want to simply remove the use of returns so that we can address the points above.