Bitwise left-shift and right-shift operators

First open-source contribution here. :smiley: I started on bitwise left-shift and right-shift operators as suggested in #60. Have tried to follow the code conventions from across the repository. Do let me know if I could make it better.

Note: I have implemented the shifting without explicitely converting BigInt to binary. Conversion cannot be avoided for other binary operators, though, so need to convert and store once during initialization and once after any binary operation.