Optimize CharIndices offset consumers

[SomeFlyingThing]

Hint that it doesn’t exceed Isize max

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[hanna-kruppe]

I'm going to go ahead and close this, as it's adding an assume without good motivation or benchmarks, which we're generally skeptical of for reasons mentioned in #161063 (comment) (and also being discussed more generally on Zulip). If you do have better motivation for this change, please open a new PR (also in line with our LLM Usage Policy).