Chanage Measured::ConversionTable from a module to a class

[kmcphillips]

Problem

The Measured::ConversionTable is built as a module where each method passes in the state.

This is pretty unnecessary. It makes the signatures messy and it's way harder to refactor as I'm trying to do for #103.

Solution

Convert it from a module to a class. Instantiate it and call #to_h. No functional change, just an internal API change.

[kmcphillips]

It's split into a three step refactor, so looking at each commit individually can make it clearer.