major pinned-vec 2.0.0 revision

• SplitVec implements "orx_pinned_vec::PinnedVec" version 2.0.
  • pinned elements guarantee is formalized and documented,
  • unsafe methods such as clone or insert are now safe. pinned vector on its own cannot build inter-element references; hence, these methods are not unsafe. this responsibility is passed to the struct enabling these references, namely, orx_selfref_col.
  • in addition to being a marker trait, this crate now provides test_pinned_vec function which is essential in asserting that a pinned vector implementation satisfies the required guarantees.
• Clone is implemented.
• Debug is implemented.
• Index-free iterator and reverse iterator implementations.