[performance] minimise log field allocations

[NyaaaWhatsUpDoc]

• moves slice functions to separate internal/util/xslices package to fix an import cycle
• adds new xslices.AppendJust() and xslices.GrowJust() functions to grow a slice in similar manners to append() and slices.Grow() but ONLY by the necessary number of elements, not any extra as normally happens in those functions typically
• makes use of xslices.AppendJust() and xslices.GrowJust() in our log package

[tsmethurst]

Looks good to me :)