pybind: add cap_bound

[jmr]

S2Cap will be wrapped soon, so the cap_bound methods can be added.

#646

@deustis

[Shaurya2k06]

Happy to pick this up. Looks like the wiring already existed in f895a59 on #646 before it got pulled back out in ebb7012 for scope reasons, I'll base the S2Cell.cap_bound() binding on that, updated for the get_ prefix drop from the later review round, plus the s2cell_bindings to s2cap_bindings BUILD dep and a matching test in s2cell_test.py.

cc @jmr

[Shaurya2k06]

Let me know if I've correctly identified this bug, and if its a required change!
cc @jmr @deustis

[deustis]

@Shaurya2k06, happy for the help! I do have a draft PR for this over here:
deustis#11

The strategy there is to bind S2Region and declare it as the base class for S2Cell, S2LatLngRect, and S2Cap.

However this is currently blocked on #646

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Edit: #646 just landed

[Shaurya2k06]

Thanks for the feedback! I know your S2Region base-class PR will end up binding this too, through inheritance, so once deustis#11 lands, #654's .def() line becomes redundant. Rather than block a live bug fix on a bigger unreviewed refactor, I'd like to suggest implementing #654 now and let you drop the now-duplicate line when you rebase on top of master. However, I'm happy to close the PR and let you take the reigns if you like.

[deustis]

Oh, is there a live bug here? I didn't realize. By all means we can ago ahead with #654 if that's the case (I thought this was just a TODO).

Separately, I just put up #655 for the S2LatLngRect bindings, which would be the final pre-req before deustis#11