Remove the 'close' and 'opened' fields.#8

Hi, beefsack. I think the `closed` and `open` fields are a bit of a contradiction, so I try to keep it simple. And I removed the `head.Remove` method, which gave a slight performance improvement. ## Benchmark ### Original ```shell $ go test -benchmem -benchtime 3s -bench . astar goos: windows goarch: amd64 pkg: astar cpu: Intel(R) Core(TM) i5-9600KF CPU @ 3.70GHz BenchmarkLarge-6 2955 1188135 ns/op 542940 B/op 6652 allocs/op PASS ok astar 3.669s ``` ### After ```shell $ go test -benchmem -benchtime 3s -bench . astar oos: windows goarch: amd64 pkg: astar cpu: Intel(R) Core(TM) i5-9600KF CPU @ 3.70GHz BenchmarkLarge-6 3030 1157325 ns/op 516254 B/op 6652 allocs/op PASS ok astar 3.665s ```