Fix for bug 133489: compiler leaks memory

Two different but related problems:

1. PySymtable_Free() must explicitly DECREF(st->st_cur), which should
always point to the global symtable entry.  This entry is setup by the
first enter_scope() call, but there is never a corresponding
exit_scope() call.

Since each entry has a reference to scopes defined within it, the
missing DECREF caused all symtable entries to be leaked.

2. The leak here masked a separate problem with
PySymtableEntry_New().  When the requested entry was found in
st->st_symbols, the entry was returned without doing an INCREF.

And problem c) The ste_children slot was getting two copies of each
child entry, because it was populating the slot on the first and
second passes.  Now only populate on the first pass.