Add .cartesian_product() adaptor to use the Product struct
Add .cartesian_product() adaptor to use the Product struct

As suggested by alexchandel.

.product() is already used by multiplicative iterator, unfortunately.

fixes #10

rust-itertools/itertools,imp/rust-itertools,tempbottle/rust-itertools,flo-l/rust-itertools,stepancheg/rust-itertools,DanielKeep/rust-itertools,Wilfred/rust-itertools