The model is revised

The crate is converted into no_std.

Although the core structure of defining a self referential collection via the expressive Variant definition is kept, a major refactoring is applied.

Lifetimes are removed from the collections which simplifies the api significantly. The safety is provided internally by NodePtr and NodeIdx.

Memory reorganization is left to the implementors through the MemoryReclaim trait.

Example implementations of singly and doubly linked lists are provided.