Undo change from list to dict for handling varnames, consts, etc.

Undo change from list to dict for handling varnames, consts, etc.

As the doc string for _lookupName() explains:

    This routine uses a list instead of a dictionary, because a
    dictionary can't store two different keys if the keys have the
    same value but different types, e.g. 2 and 2L.  The compiler
    must treat these two separately, so it does an explicit type
    comparison before comparing the values.