v0.4.1

Added

• New feature: alloc

Changed

• Dropped the Peripheral Access Crate implementation of the audio driver in favour of the embedded_hal
  implementation.
• Refactored the crate to make the requirement for rust nightly and the alloc
  crate optional. See the audio_* examples for more details.
• Refactored the crate to remove implicit definitions of #[interrupt] handlers in favour of explicit declaration
  in program code. See the audio_* examples for more details.