add DataType::element_stride()

[cyrush]

strides are in bytes, but per element strides are convenient when adapting to many apis.

caveat: its possible that byte-strides may not be element aligned -- also figure out how to identify this case

[cyrush]

DataType::element_stride();
DataType::is_stride_element_aligned();
DataType::is_stride_aligned(index_t nbytes);