Consider struct types for TickitPosition and TickitOffset

The discussion on whether Tickit APIs are line- or column-major can be almost entirely averted, if every function that takes a notional "position" or "offset", instead took a simple two-int struct of TickitPosition(TickitPos?) or TickitOffset. This moves the decision out of the immediate API.

Perhaps then to avoid quite so much typing at callsites, we could provide some macros that create the obvious structures from two integer arguments.

#define TICKIT_POS_LC(l,c) ((TickitPos){.line = l, .col = c})
#define TICKIT_POS_XY(x,y) ((TickitPos){.col = x, .line = y})

might be one solution.