start pruning reimplementation in cmd/pruning subpackage

cmd/pruning/keep_helpers.go

@@ -0,0 +1,17 @@
+package pruning
+
+func filterSnapList(snaps []Snapshot, predicate func(Snapshot) bool) []Snapshot {
+	r := make([]Snapshot, 0, len(snaps))
+	for i := range snaps {
+		if predicate(snaps[i]) {
+			r = append(r, snaps[i])
+		}
+	}
+	return r
+}
+
+func shallowCopySnapList(snaps []Snapshot) []Snapshot {
+	c := make([]Snapshot, len(snaps))
+	copy(c, snaps)
+	return c
+}