major refactoring applied

The following major revision is applied to the pinned vector:

• 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.