CellID.AllNeighbors: Allocate memory in one go

[fx408]

Pre-allocates memory to avoid triggering expansion, and enhances performance.

• Before：

BenchmarkAllNeighbors/level12         	 3699290	       322.9 ns/op	     120 B/op	       4 allocs/op
BenchmarkAllNeighbors/level14         	 1699875	       700.6 ns/op	     504 B/op	       6 allocs/op
BenchmarkAllNeighbors/level16         	  594056	        2091 ns/op	    2040 B/op	       8 allocs/op

• After：

BenchmarkAllNeighbors/level12         	 5084275	       254.1 ns/op	      64 B/op	       1 allocs/op
BenchmarkAllNeighbors/level14         	 2295961	       510.6 ns/op	     160 B/op	       1 allocs/op
BenchmarkAllNeighbors/level16        	  786132	        1610 ns/op	     576 B/op	       1 allocs/op

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[panmari]

Thanks for your contribution! Just a small style nit. Did you actually see this cause a performance issue on your end?

[jmr]

Note there are some failing checks.

[jmr]

Not sure what these values buy over 10.1, 10.1 or 10.0, 10.0, but we can change it later.

[fx408]

Not sure what these values buy over 10.1, 10.1 or 10.0, 10.0, but we can change it later.

I think there is no difference.

[jmr]

Thanks! Did you just happen to notice this, or was it actually causing a performance problem?

It would be great if you could look for other instances of the same problem.