Serde integration for nalgebra-sparse

[Andlon]

nalgebra-sparse currently does not implement Serialize or Deserialize. Serialization should ideally be implemented for all major storage types (COO, CSR, CSC). These implementations should be hidden behind a feature flag, like nalgebra.

Note that deserialization requires some care, and simply slapping derive(Deserialize) is not sufficient. The reason is that the sparse matrix formats have some invariants that must be verified in order for subsequent use to remain sound.