Add cusparse safe wrappers for sparse-dense format conversions

[g-ampo]

result.rs already has bindings for cusparseSparseToDense_bufferSize, cusparseSparseToDense, cusparseDenseToSparse_bufferSize, and cusparseDenseToSparse (added in #569).

safe.rs currently wraps spmv() and spmm() but not the format conversion APIs. Adding sparse_to_dense() and dense_to_sparse() to CudaSparse with automatic workspace management would complete the generic API coverage.