Reintroduce nnz and nonzeros as discussed in #6769
Deprecate nfilled for SparseMatrixCSC.
Deprecate nonzeros for StridedArray and BitArray.
Deprecate nnz for StridedArray.
Update documentation for structural nonzeros.
Update NEWS
Merge pull request #6963 from JuliaLang/vs/nnz

RFC: Reintroduce nnz and nonzeros