Make builder naming and behaviour more consistent

[mattxwang]

I'm breaking up the refactor into several (squash-mergeable) PRs, to make it easier to view diffs and future git blames.

This PR touches many files but is mostly just renaming and code motion. The biggest naming changes are:

• Manager -> Builder
• mgr/man -> builder
• when from_cnf is an instance method, convert to compile_cnf

For BDDs,

• I've removed the duplicate BddPtr and BddManager. There is now one concrete BddPtr (the old one in bdd.rs), a BddBuilder trait, and a concrete RobddBuilder. The functionality is split up between the trait and the concrete implementation, which now live in different files.
• I've moved the BddPlan to its own top-level module; in the future, this will become a BottomUpPlan
• I've moved compilation from various formats (ex compile_cnf) to be part of the BddBuilder trait
• I've removed a panic-based node_ref fn and changed call sites to pattern match
• when possible, the builder now uses the BottomUpBuilder::eq instead of bdd_eq

For SDDs,

• I've renamed SddManager -> SddBuilder (and implementing structs)
• when possible, the builder now uses the BottomUpBuilder::eq instead of sdd_eq

Along the way, I've also changed some minor items to be closer to Rust conventions, and removed some useless tests (ones that are assert!(true).