0.6.0

CHANGES:

• Added a futex-like Awaitable abstraction as the picos_std.awaitable
  library (@polytypic)
• Changed the core Picos library to be internally built from a single .ml file
  (@polytypic)
• Optimized heap and stack usage of fibers and resource cleanup mechanisms and
  added workarounds for compiler generated space leaks due to closures
  (@polytypic)
• Added lastly as a safe alternative to Fun.protect (@polytypic)
• Workarounds for the Uri library not being threadsafe (@polytypic)
• Fixed to raise proper error when Picos_io_select has not been configured
  properly (@polytypic)
• Forbid cancelation propagation during release calls in the
  picos_std.finally library (@polytypic)
  • This is a change in behaviour and could be seen as a breaking change, but it
    should really be considered a bug fix.
• Renamed (Ivar|Stream).poison to (Ivar|Stream).poison_at and added
  (Ivar|Stream).poison with optional ?callstack:int (@polytypic)