Re http://bugs.python.org/issue4315

The symbol table used the same name dictionaries to recursively
analyze each of its child blocks, even though the dictionaries are
modified during analysis.  The fix is to create new temporary
dictionaries via the analyze_child_block().  The only information that
needs to propagate back up is the names of the free variables.

Add more comments and break out a helper function.  This code doesn't
get any easier to understand when you only look at it once a year.