Release 0.15.0

• Moved World to shred, added WorldExt trait for Specs functionality (#550)
• Add UuidMarker for UUID <-> Entity mappings (#584)
• Implement Join on BitSetLike trait object (#599)
• Expose inner field of AntiStorage (#603)
• Remove fnv in favour of hashbrown (#606)
• Reexport hibitset, rayon, shred and shrev (#606)
• Reexport shred_derive::SystemData when shred-derive feature is enabled (#606)
• Reexport specs_derive::{Component, ConvertSaveload} when specs-derive feature is enabled
  (#606)