Enforce UnitSort tag precision per spec. (#111)

Author: GabrielDosReis

include/ifc/file.hxx

@@ -132,9 +132,14 @@ namespace ifc {
         Partition,  // Module interface partition unit.
         Header,     // Header unit.
         ExportedTU, // Translation unit where every declaration is exported, scheduled for removal.
+        Unused0,    // Reserved for future use.
+        Unused1,    // Reserved for future use.
+        Unused2,    // Reserved for future use.
         Count
     };
 
+    static_assert(index_like::tag_precision<UnitSort> == 3);        // Per current specification.
+
     struct UnitIndex : index_like::Over<UnitSort> {
         using Base = index_like::Over<UnitSort>;