This PR makes two changes, both suggested by @fitzgen in #36:
1. It updates the top-level description of the analysis to more
simply and accurately describe the analysis lattice.
2. It modifies both the `CheckerValue` and `CheckerState` types to be
enums with separate arms for the top/universe value, and adds helpers
as appropriate to update the values. There should be no functional
change; this update just makes the meet-functions and updates more
clear, and makes a bad state ("top" but with values) unrepresentable.
Closes#36.
a369150213