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.

Documentation revised wrt LinkedList:1.0 impl

Merge pull request #9 from orxfun/documentation

documentation revised wrt LinkedList impl

`self_referential_elements` module

self_referential_elements

orx_pinned_vec::self_referential_elements module is created. This method defines all traits required to conveniently, efficiently and simply define inter-element references of a self-referential-collection.

There are three variants for each of next and prev references:

• single
• multiple references with dynamic sized (vec)
• multiple references const capacity (const array)

Each node can define required traits.

For instance a linked list node would only require the single-next. A doubly-linked-list node would additionally implement single-prev trait.

A tree on the other hand can implement single-prev to define the parent of a node and a dynamic sized vec to define its children. This can be replaced by a constant capacity array for a binary search tree, for instance.

Also

• iter_mut is added to the trait definition.