Back LongSequence and LongSubSeq by RefVector #349

In MemoryViews, I am considering making a type

struct RefVector{T} <: DenseVector{T}
    ref::MemoryRef{T}
end

whose length is then obtained from the remainder of the Memory. We can replace existing Vector with this, which saves a memory indirection. Note that we can still resize by making the RefVector field in LongSequence non-constant and replacing it.

This requires Julia 1.11, so maybe we want to wait until LTS has been bumped