Serde integration for nalgebra-sparse

`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.