added truncate and file_size
accept iolist as file name to open
properly indented init

Which was "misformatted" due to it being stolen from Filipe's snappy
nif.
use sync instead of async for now
return {ok, Ref} from open and dup

To make them more couch_file compatible
handle eof from pread compatibly with efile

And we also now return {ok, Bin} for same reason.
added runtime switch between sync and async
added simple bench for async vs. sync vs. efile