Code cleanup surrounding letter/latin/s.ptl.

[jmcwilliams403]

This consolidates the different SShapeImpl functions into four main ones:
SStrokeImpl, RevSStrokeImpl, SmallSStrokeImpl, and RevSmallSStrokeImpl
with things like SCurlyTail and SmallSStrokePhoneticRight being implemented as sb arguments for SmallSStrokeImpl. In effect, this actually makes the different non-canonical s-derived composites more consistent with each other.

SStrokeAlt is left alone because the diff was getting very large, but I made sure its definition came last in the set instead of having RevSmallSStroke etc. dangling at the end. If it wouldn't make the diff so messy as a result, I would have actually moved SStrokeAlt to be next to the blackboard bold letters that use it.

Also unify overshoots (or also "undershoots" in some of these usages) of rev-S/rev-s shapes with that of normal S/s.

Ordinary ASCII S/s is unchanged.