Create an internal source manager decorator for caching

[sdboyer]

In order to keep the main sourceManager as agnostic to the particular project it's solving for as possible, we need a separate SourceManager decorator-style object that is created internally on each Solve(), and can become responsible for handling things like #13 .

This architectural improvement will let us keep the solve-specific logic separate from the system-tracking logic. Another example would be caching the sorted, computed version list based on the solve direction.

[sdboyer]

This has architectural value beyond just the performance. It'll help keep the layers separate with #10 , for example. So, making it part of MVP.