Find nearby cycle points without iterating from sequence start

[hjoliver]

In several places in the new ISO8601 cycling implementation we find an on-sequence point at or near to some other point X by iterating from the sequence start point until at or beyond X. This is not efficient if X is a long way beyond the sequence start point. It should be (I hope) possible to find a nearby on-sequence more directly, like in the old (otherwise vastly inferior!) cycling modules. See #1044 (comment), #1044 (comment), and #1044 (comment).

[benfitzpatrick]

This is causing some problems, so I'd like to address this in the next couple of weeks.

[hjoliver]

@benfitzpatrick - should this be closed now, or do you think we can still do better in the future? (in #1850 you wrote "This addresses part of #1053".)

[benfitzpatrick]

I think we can close it - as it's written, it's addressed by #1850.