stats: create an enable phase, and a prepare phase.

stats: create an enable phase, and a prepare phase.
Enable more or less takes the place of check, but also allows stats to
do some other configuration.  Prepare moves all of the code that readies
a stat for dumping into a separate function in preparation for supporting
serialization of certain pieces of statistics data.
While we're at it, clean up the visitor code and some of the python code.