Remove the 'close' and 'opened' fields.

[aitsuki]

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

$ 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

$ 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