Refactor BitsTypeDefinition.Bit and its implementations

Bit as such is not a SchemaNode, hence it should derive only from DocumentedNode.WithStatus.

Furthermore BitStatement/BitEffectiveStatement should not be using QName as their argument, only the string (which should be validated to conform to QName localName rules, but it's still a string).

Finally the actual implementation is wrong, as we are translating BitEffectiveStatements into BitImpl, which is a bit wrong - as the statement world does not match the type world. This should be fixed by BitEffectiveStatement implementing Bit - and its buildEffective() method needs to create implicit Position statements as appropriate.