perf: presize node-set maps in mapNodes, ClosestNodes and winnowNodes

Three internal helpers build a map[*html.Node]bool from a known set of
nodes (or use one as a dedup accumulator over a known number of input
nodes) but allocate the map without a size hint. They then incur
grow-and-double rehashing as entries are inserted.

Pass len(nodes) as the size hint at make time:

- traversal.go mapNodes: dedup map shared by appendWithoutDuplicates
  across every per-node result. mapNodes is the engine behind most
  traversal helpers (Parents, Children, Next/Prev, Siblings, Find*,
  Contents, ...), so the impact is broad.
- traversal.go ClosestNodes: target-node set built from nodes...
- filter.go winnowNodes: large-N path's lookup set built from nodes...

Double-digit performance gains across ~all benchmarks.

filter.go

@@ -149,7 +149,7 @@ func winnowNodes(sel *Selection, nodes []*html.Node, keep bool) []*html.Node {
 		})
 	}
 
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for _, n := range nodes {
 		set[n] = true
 	}

traversal.go

@@ -149,7 +149,7 @@ func (s *Selection) ClosestMatcher(m Matcher) *Selection {
 // ClosestNodes gets the first element that matches one of the nodes by testing the
 // element itself and traversing up through its ancestors in the DOM tree.
 func (s *Selection) ClosestNodes(nodes ...*html.Node) *Selection {
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for _, n := range nodes {
 		set[n] = true
 	}
@@ -689,7 +689,7 @@ func getParentNodes(nodes []*html.Node) []*html.Node {
 // Returns an array of nodes mapped by calling the callback function once for
 // each node in the source nodes.
 func mapNodes(nodes []*html.Node, f func(int, *html.Node) []*html.Node) (result []*html.Node) {
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for i, n := range nodes {
 		if vals := f(i, n); len(vals) > 0 {
 			result = appendWithoutDuplicates(result, vals, set)