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Add a generic vector type to the library, with some special features:
- the elements are embedded in the vector, and not pointers
- each element must include an int field that is used as a "key"
- the element has a "null" value for its key, used to indicate that this
element of the vector is null.
- add/remove functions provide access to a new element/free it from the
vector, but don't "destroy" it.
- resize on add is exponential.

This patch includes implementation and unit test.