Segment/Chunk usability

[mpilquist]

This PR includes:

• Segment operations which force evaluation of any elements have been moved to Segment.Force, resulting in syntax like s.force.toVector, s.force.run, s.force.unconsChunk. The primary motivation here is to be clear when evaluation occurs.
• Segment no longer overrides equals and hashCode as computing these values necessarily forces the segment, which may hang if the segment is infinite.
• Chunk is no longer a subtype of Segment. A chunk can be lifted to a segment via Segment.chunk(c) or c.toSegment. By avoiding subtyping, users know that chunk operations are always strict. This is a significant improvement over the old API where an operation on a Chunk may be strict (if defined directly on Chunk), lazy, or partially eager and partially lazy.
• Chunk#{head,last} were changed to return Option[O] -- the only unsafe operation on Chunk is apply now.