Marshaler and Unmarshaler

I'd like to add marhsalling and unmarshalling to this package. I'd implement [encoding.BinaryMarshaler](https://pkg.go.dev/encoding#BinaryMarshaler) and the [`smallBytes` format](https://github.com/tdunning/t-digest/blob/main/core/src/main/java/com/tdunning/math/stats/MergingDigest.java#L883) from the Java reference implementation. Alternatively, I could add `AsBytes` and `AsSmallBytes` method. Is there any interest?

I could also go straight to Protobuf if there's interest. E.g. DDSketch Go comes with a Protobuf definition.

First review the package API and the linked Java MergingDigest.java smallBytes implementation, then compare the BinaryMarshaler, AsBytes/AsSmallBytes, and Protobuf options described in the issue. Confirm the serialization direction with maintainers; done would mean an agreed format and implementation with compatibility expectations documented.

Written by the indexing model from the issue text.