V2 API using Go 1.23 iterators

[bobg]

This PR updates hashsplit to V2 of the API.

This is a very different API from V1, and dramatically simpler. It is designed around the range-over-function feature introduced in Go 1.23, a.k.a. iterators.

API changes aside, the hashsplit boundaries and hashsplit-tree shapes computed by this module are the same as before.

[github-actions]

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This PR requires an increase in your module’s major version number.
If the new major version number is 2 or greater,
you must also add or update the version suffix
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no new version of package github.com/bobg/hashsplit
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