Rust Embedded-HAL PCF8591 Driver

For interfacing with the PCF8591 8-bit A/D and D/A converter. Currently only handles reading a single single-ended ADC at a time.

Example

With the PCF8591 connected to a raspberry pi:

```rust use linuxembeddedhal::I2cdev; use pcf8591::*;

pub fn main() { let i2c = I2cdev::new("/dev/i2c-1").expect("can open i2c device"); let mut adc = PCF8591::new(i2c, PCF8591DEFAULTADDRESS);

loop {
    let a0 = adc.read(PCFADCNum::A0).expect("can read ADC0");
    println!("a0: {}", a0);
    std::thread::sleep(std::time::Duration::from_secs(1));
}

} ```