Implement subtype predicate parsing.

The subtype given for variables are now properly parsed into the entire system.
Currently they are not actually used when generating the checker statements.
The expressions are fully parsed expressions, ready for output however.

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@10854 57e6efec-57d4-0310-aeb1-a6c144bb1a8b