Measure the usefulness of check against decomposition_passthrough_bound

[hsivonen]

Decomposition::delegate_next_no_pending() checks the character against decomposition_passthrough_bound. Measure if this is actually an optimization in the absence of a supplement: I.e. measure if the trie bails out quickly on its own.

[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.