feat: Use enum for duty ratio

[yutannihilation]

TonePattern::with_duty() takes a 2-bit value that represents the duty ratio. But, I think it's hard to specify the ratio by a raw value. This pull request adds an enum to represent the four types of duty ratios (12.5%, 25%, 50%, 75%) and use it as the argument of with_duty().

pub enum DutyRatio {
  _12_5 = 0,
  _25 = 1,
  _50 = 2,
  _75 = 3,
}

References:

• https://www.coranac.com/tonc/text/sndsqr.htm
• https://problemkaputt.de/gbatek-gba-sound-channel-1-tone-sweep.htm