* add API for iterating over custom HTTP headers
* remove `trailing` flag from std.http.Client.parse. Instead, simply
don't call parse() for trailers.
* fix the logic inside that parse() function. it was using wrong std.mem
functions, ignoring malformed data, and returned errors on dead
branches.
* simplify logic inside wait()
* fix HeadersParser not dropping the 2 read bytes of \r\n after a
chunked transfer
* move the trailers test to be a std lib unit test and make it pass
