Output `boolean` instead of `ternary` if limited to true/false

Add subset information to types

Add enum IntegerSubset

Convert to ref semantics

Updated plan

Reduce simple functions to their returned value

Plan

Reduce references to immutable variables

Add struct VariableDeclaration

Reduce known adjustment to its returned value

Improve tests

Test invariants

Inline helper functions

Rename `unset` -> `ambiguous`

Rename Ternary -> TruthValue