implementation of the PN532 I2C spec in rust

useage:

```rust use nfc::Pn532;

pub fn main() { let mut nfc = Pn532::open("/dev/i2c-0").unwrap(); let fwv = nfc.getfirmwareversion().unwrap(); println!("nfc firmware version: {}.{}", fwv.1, fwv.2);

nfc.setup().unwrap();
println!("{:x?}", nfc.list(std::time::Duration::from_secs(1)).unwrap());
nfc.powerdown().unwrap();

}

```