helper to grab a fn that is passed in as an argument
the function to search for
the environment to start searching in
this is based on Hadley Wickham's response to an SO post: http://stackoverflow.com/questions/14183766/match-fun-provide-error-with-functions-defined-inside-functions with some minor modifications
fn, if fn is already a function; otherwise, the first function found in env or one of its parents whose name is fn
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