Clone, Index and IndexMut traits implemented

• Thread safe Clone method is implemented.
• Index and IndexMut traits are implemented for usize indices.
  • Due to possibly fragmented nature of the underlying pinned vec, and since we cannot return a reference to a temporarily created collection, currently index over a range is not possible. This would be possible with IndexMove (see rust-lang/rfcs#997).
• Debug trait implementation is made mode convenient, revealing the current len and capacity in addition to the elements.
• Upgraded dependencies to pinned-vec (3.7) and pinned-concurrent-col (2.6) versions.
• Tests are extended:
  • concurrent clone is tested.
  • in addition to concurrent push & read, safety of concurrent extend & read is also tested.
• boxcar::Vec is added to the benchmarks.