Gates

2 input AND, OR, NAND, NOR, XOR and XNOR gates and NOT gate
RTL model using operators and structural model using gate primitives

File structure example
> nand
>> nand_struct.v
    nand_struct_tb.v
    nand_rtl.v
    nand_rtl_tb.v

where `_struct` is for the structural model and `_rtl` for the RTL model

This issue can be broken down and solved in multiple PRs.