Potential for FastString uniques clashing with others

Currently the getUnique implementation for FastString derives the unique from a sequential counter (starting at 0) associated with the global FastStringTable. One can argue that this is okay since the \x00 unique tag isn't used by any UniqueSupply. However, if we allocate allocate enough FastStrings we will overflow the zero-tag into regions of the tag-space used by other UniqueSupplys. This will result in unique collisions.