Add support for offsetting the phase of a signal

[viktaur]

Hi, this PR introduces a new struct and a method to specify the initial phase of a signal, represented by a value between 0 and 1.

Example:

use dasp_signal::{self as signal, Signal};

fn main() {
    let step = signal::rate(4.0).const_hz(1.0).offset_phase(0.25);
    let mut phase = step.phase();
    assert_eq!(phase.next(), 0.25);
    assert_eq!(phase.next(), 0.5);
    assert_eq!(phase.next(), 0.75);
    assert_eq!(phase.next(), 0.0);
    assert_eq!(phase.next(), 0.25);
    assert_eq!(phase.next(), 0.5);
}

Feel free to comment on any design choices that could be improved.