Change StrSet::is_subset_of to correctly handle empty sets, add some comments
Add Tset::insert_symbol
Add tests for new functions
Bump required corpus2 version to 0.1.2 due to a fixed bug there

Fixing expected interface of sets i.e. bool intersects function, not a function returning actual intersection

Adding some methods to values for use by operators, e.g. equals and is_subset_of/intersection. Some are just stubs.