flatarray vs arrayvec vs orx-pinned-vec

Implementation of a flattened iterator, that acts as if it still was an iterator of iterator

A vector with fixed capacity, backed by an array (it can be stored on the stack too). Implements fixed capacity ArrayVec and ArrayString.

`PinnedVec` trait defines the interface for vectors which guarantee that elements added to the vector are pinned to their memory locations unless explicitly changed.