move most of Vector to MemSpan

[alex-s168]

Add a new type MemSpan which represents a re-allocatable, contigious, mutable, owned collection.
It should not be used direclty, but instead serve as backing collection type for things like Vector and future collection types.
This should provide a safe interface to libc's realloc and also take care of ownership of the inner elements (when resizing or freeing)
Important for preventing errors!

(assign me please)

[andrew-johnson-4]

A Sized<x> that "takes care of ownership of the inner elements" is interesting. The problem of safely typed memory allocations with only minimal structural typing is hard but worthwhile.

Thoughts on a minimal memory model so far: https://github.com/andrew-johnson-4/LSTS/wiki/Memory-Model