This new function is much faster than ensureRange(pos, pos+1), which is a very
common case.
This speeds up the rendering of some test cases (including the Tracemonkey
paper) by 4--5%.
This is achieved by adding getBytes2() and getBytes4() to streams, and by
changing int16() and int32() to take multiple scalar args instead of an array
arg.
By checking if the data is all present before making a substream, we avoid
cases where we parse part of a stream and then throw a MissingDataException
part-way through, which forces us to later re-read the stream -- possibly
multiple times. This is a sizeable performance win for some cases when file
loading is slow (e.g. over the web).
