Allow the encoding to be generated on the fly in `solve()`

[garrison]

Summary

Fixes #9.

Details and comments

Remaining tasks:

• [ ] Add more test coverage
• [ ] Figure out a workaround for our prior need to access self.encoding.offset in solve(). offset somehow needs to make it from solve_relaxed() to solve(). It seems like solve_relaxed() almost needs to return the offset as a third tuple element ... or, we could put it in the RoundingContext, but it doesn't seem to really belong there. Potentially related to #8. Possible solution in #14.