This update will reduce the classvar mapping lists generated from 
virtual mapping files.

Previously all destination values where always extracted. Now the
EXACT_MAPPINGS section is first scanned to find all possible destination
values for every source value and then when the data is scanned only the
potential destination values are added to the classvar mapping output.

This improvement will usually give significant reductions in the size of 
the classvar_mapping file when the mapping files are automatically created
from external sources.