Fixed the checkFieldNames: no ArrayOutOfBounds exception thrown when
constructors have fields with the same name and different types.
Merge branches 'master' and 'refactor-subtype' of https://github.com/impulse-org/pdb.values into refactor-subtype
Added missing 'is<Type>()' methods.
Implemented 'isTuple()' in terms of 'isFixedWidth()'.
Fixed AliasType.isSupertypeOf(Type).
Fixed ListType.toString() and SetType.toString().
Fixed 'Type.isStrictSubtypeOf(Type)'.