Reorganize and refactor libraries

[cgranade]

As the Quantum Development Kit grows, it might be good to take a step back and try to make the API surface for the standard libraries a little bit more uniform (e.g.: consolidate the names of type conversion functions, use naming conventions to communicate what callables are operations, etc.). Similarly, we can make better use of new language features that have been introduced since Q# 0.1.

[cgranade]

This has now been released in 0.6, so closing the issue.