Following a suggestion by Asger, we track use nodes through calls to `promisify`. When we see a call to a promisified function, we introduce a new synthetic API-graph node representing the callback argument synthesised by the promisification, and track the result of the call to an `await` (or other promise resolution), which is then considered to be a use of the first parameter of the synthetic callback (the zeroth parameter being an error code, which we do not model yet).
