Most system headers contain functions (e.g. `memcpy` in `string.h`)
which aren't needed in our own header files. For the most part, our own
headers should only include types needed to declare our own types and
functions. We now enforce this so we think twice about which headers we
really need in the .h files.
We should avoid recursion, as it makes reasoning about stack growth
harder. This tool shows (currently) 4 (non-tail) recursive functions, at
least 2 of which are easy to fix.
