digital_electricity: Add universal and XOR gates

Added XOR gates and the two universal gates, namely NOR and NAND gates.
Also, added two basic levels (7 and 8) and to demonstrate the uses of
the XOR and NAND gates along with level 9 to demonstrate the use of
XOR gates using 3 inputs

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>