add docstrings

[mattjj]

Docstrings for some key functions and data structures, especially Node and Box, might help. Any suggestions for specific places to add docstrings?

[dustinvtran]

One that came up today: subvals. Reading through it, I guess it stands for "substitute values"?

In general, some of the shorthand can be difficult to trace (no pun intended).

[dustinvtran]

Another is primitive, which wraps f_raw followed by wraps which wraps f_wrapped and is parameterized by f_raw. It's not clear why there's nested decorators until you read and grok all the logic.