Split generic tests into separate package

We have an impressive battery of tests which are generic in vector type. I propose splitting them into separate package to benefit anyone who defines instances for vector type classes (me). It turns out creating nontrivial instance *is* error prone. I was writing storable vectors with stride (vector elements are every n-th in buffer)

This will entail creating another package `vector-test` which exposes generic tests and moving vector's test suite there. @lehins @Bodigrim what's you opinion on that?