Move nested classes out of Comparisons class

[nschiefer]

Currently, most Comparison implementations are nested inside of the Comparisons class, which is effectively used as a namespace. This has gotten a bit unwieldy.

Although it's a large breaking change, I'd like to refactor Comparisons into a package with a bunch of non-nested classes.

[nschiefer]

Tentatively placing this in 3.0 since it's a big breaking change.