Yosys JSON netlist serde structures

Read/write Yosys JSON files in Rust using serde.

Reading

```rust // from a byte slice let result = Netlist::from_slice(...).unwrap();

// from an io::Read let result = Netlist::from_reader(reader).unwrap(); ```

Writing

rust let mut netlist = Netlist::new("Super cool HDL tool"); ... let json = netlist.to_string().unwrap();