Let OneElement be AbstractSparseArray?

[putianyi889]

For general purposes where the type doesn't propagate, it should be promoted to a concrete sparse array. Example:

• Optimize + for OneElement #260

[jishnub]

Could you make a PR?

On second thoughts, this would require a hard dependency on SparseArrays

[putianyi889]

this would require a hard dependency on SparseArrays

create a new package StaticSparseArrays?

looks like SparseArrays.jl is mostly SparseMatrixCSCs.jl and there is not much generic interface...