v0.2.0

Release 0.2.0

• Includes Semaphore API to acquire multiple permits in a single call
• Introduces alloc feature and enables semaphore without it, i.e.,
  Semaphore now requires zero allocations.
• Enables non-allocating oneshot channel uses without alloc feature.
• Document panic behavior when creating bounded channels with zero capacity.
• Adds from_iter APIs for bounded and unbounded channels
• Adds bounded::[Channel|Sender|SenderRef]::unbounded_send API.
• Adds Semaphore::outstanding_permits for accounting handed out permits

Breaking Changes

• Renames Semaphore::[try_]acquire_one to [try]_acquire
• Introduces split alloc and std features, bounded and unbounded no
  longer exist without the alloc feature enabled,
  oneshot::Channel::into_split likewise requires alloc.