> one never needs to look at more than two adjacent code points to tell 
whether or not a grapheme break will occur between them, so this ought 
to be pretty efficient. 


That note is outdated (and has been outdated since Unicode 9). The regional indicator rules (GB12 and GB13) and the emoji rule (GB11) require arbitrary lookbehind (though thankfully not arbitrary lookahead).

I think the ideal API surface is an iterator and nothing else. Everything else can be derived from the iterator. It's theoretically possible to expose an is_grapheme_break that's faster than just iterating -- look at the code in unicode-segmentation's _reverse_ iterator to see how -- but it's going to be tricky to get right. Building the iterator on top of is_grapheme_break is not a good idea.