Changed the implementation strategy for collections.namedtuple() to
substantially reduce the use of exec() in favor of precomputed methods. As a
result, the *verbose* parameter and *_source* attribute are no longer
supported.  The benefits include 1) having a smaller memory footprint for
applications using multiple named tuples, 2) faster creation of the named
tuple class (approx 4x to 6x depending on how it is measured), and 3) minor
speed-ups for instance creation using __new__, _make, and _replace.  (The
primary patch contributor is Jelle Zijlstra with further improvements by
INADA Naoki, Serhiy Storchaka, and Raymond Hettinger.)
