refactor: reorganize theta family mods and add key-only sketch views

[ariesdevil]

Summary

• Group the shared Theta, Theta, and Tuple implementations under theta_family.
  • Preserve the existing public module paths through re-exports.
• Introduce key-only sketch views for operations that only consume retained hashes.
  • Update Jaccard similarity and the B side of A-not-B to avoid cloning Tuple summaries.
  • Keep full retained-entry views for operations such as union and intersection.

The theta family becomes:

datasketches/src/theta_family/
├── common/
├── theta/
├── tuple/
└── mod.rs

[tisonkun]

Generally LGTM.

But I'd still prefer:

theta
thetacommon
tuple

.. structure.

But I can submit a follow-up PR to demonstrate that structure tomorrow.

[tisonkun]

Now I know you use a theta_family mod to keep several methods pub(super).

[tisonkun]

I pushed #179 to rename theta_family to thetafamily. It's not exported so it makes no difference from user interfaces, but keep our top-level modules no separator which provides a stable style (my bias, yes).

[ariesdevil]

But I'd still prefer:

theta
thetacommon
tuple

.. structure.

Actually, I like the original structure too, but I don't want to put thetacommon at the top level, so I came up with a compromise.