Consider deferring trie lookup for characters below U+0300 in the NFC iterator case

[hsivonen]

#2378 optimizes away trie lookups when NFC passthrough succeeds. However, that's only with contiguous buffers. With the iterator formulation, characters that are equal or above decomposition_passthrough_bound but below composition_passthrough_bound could potentially avoid a trie lookup, but currently a trie lookup is made anyway.

Figure out if there's a reasonably-manageable way to defer the trie lookup in the iterator case.

[sffc]

@hsivonen What would be an appropriate milestone for this issue?

[hsivonen]

This isn't urgent, so it's kinda weird to assign a milestone. Since this doesn't need to be in 1.0 and not necessarily in next, either. I put 1.2 as the milestone.