feat(skiplist): Add sorted skiplist builder

[ahsanbarkati]

Add a Builder type in skiplist package which can be used to insert
sorted keys efficiently.

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This change is 

[manishrjain]

Reviewed 3 of 3 files at r2.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @ahsanbarkati)

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skl/skl.go, line 550 at r2 (raw file):

	if debug {
		if len(b.prevKey) > 0 && y.CompareKeys(k, b.prevKey) <= 0 {
			panic(fmt.Sprintf("new key: %s <= prev key: %s\n", y.ParseKey(k), y.ParseKey(b.prevKey)))

overflow

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skl/skl.go, line 563 at r2 (raw file):

	for i := 0; i < height; i++ {
		node := b.prev[i]
		node.tower[i] = s.arena.getNodeOffset(x)

can move this outside.