Use consistent naming for value type attributes.

Use consistent naming for value type attributes.

	* python/libstdcxx/v6/xmethods.py (ArrayWorkerBase): Rename _valtype
	to _val_type.
	(ArraySizeWorker, ArrayEmptyWorker): Ditto.
	(ArrayFrontWorker, ArrayBackWorker): Ditto.
	(ArrayAtWorker, ArraySubscriptWorker): Ditto.
	(DequeWorkerBase): Rename elemtype to val_type.
	(ForwardListWorkerBase): Rename _elem_type to _val_type.
	(ForwardListFrontWorker): Ditto.  And rename elem_address to
	val_address.
	(ForwardListMethodsMatcher): Rename elem_type to val_type.
	(VectorWorkerBase): Rename _elemtype to _val_type.

From-SVN: r222596