Fix next on SourceFile

next(iterable) is supposed to return the next thing in the sequence;
here, that's a bunch of zeros. However, this was returning a generator
that yielded one item: a bunch of zeros. This broke direct
benchmarking.

Change-Id: I2fca4795fc9b3d1637c72616a52a60482d6e853e

swiftbench/bench.py

@@ -101,8 +101,8 @@ class SourceFile(object):
         if self.pos >= self.size:
             raise StopIteration
         chunk_size = min(self.size - self.pos, self.chunk_size)
-        yield '0' * chunk_size
         self.pos += chunk_size
+        return '0' * chunk_size
 
     def read(self, desired_size):
         chunk_size = min(self.size - self.pos, desired_size)