Add stack vector implementation

It can be a separate implementation or specific allocator implementation for std::vector.

It may use an array of fixed size and variable length.

https://stackoverflow.com/questions/354442/looking-for-c-stl-like-vector-class-but-using-stack-storage

It can be useful to make github.com/linksplatform/Data.Doublets work faster.

To use it, it should give better performance than std::vector.

The issue names no repository files or tests; begin by locating the collection implementations and how linksplatform/Data.Doublets uses them. Compare any stack-backed design with std::vector and define completion as demonstrating better performance than std::vector for the intended use.

Written by the indexing model from the issue text.