Lazy loading

[mgree]

Lazily populate the FS. The current implementation passes tests but has hefty technical debt.

• Rename fs.rs into eager.rs. 24d9704
• Use the new Nodelike parsing/unparsing logic to simplify loading for eager::FS, too. 9268a77
• Let lazy loading rewrite input into a different output format by calling resolve_node to fixpoint. aba72bc
• CLI flags for controlling laziness. df22c78

It's tempting to have eager mode work with the lazy FS by just forcing resolution during startup. There would be some overhead, though. How much? We can do benchmarking on three modes: lazy, lazy with --force-early, and the existing eager. dd152f1 shows that lazy with --force-early is on par with eager. So once we can have lazy outputting in whatever format, we can drop eager entirely.