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.

src/python/m5/core.py

@@ -27,7 +27,14 @@
 # Authors: Nathan Binkert
 
 import internal
+from internal.core import initAll, regAllStats
 
 def setOutputDir(dir):
     internal.core.setOutputDir(dir)
 
+def initAll():
+    internal.core.initAll()
+
+def regAllStats():
+    internal.core.regAllStats()
+