feat: improve Expression::simplify

[kalzoo]

Closes #208

This should be considered proof-of-concept quality at this point.

Ideally, we'd be able to use a library to implement expression simplification; this is reinventing an oft-reinvented wheel. However, we do need some minimal utility here for a particular use case, and so I don't want perfect to be the enemy of minimally good.

Questions for reviewers:

• Is there a library you'd recommend for use here?
• Are there clever patterns we can put to work?
• How can we prevent unbounded recursion? At a minimum, we could pass a depth argument or could recognize when simplification is recursing without modification

[kalzoo]

Superseded by #210