Speed up GetPage

Speed up GetPage

```
name                             old time/op    new time/op    delta
GetPage-10                          413ns ± 0%     287ns ± 1%  -30.47%  (p=0.029 n=4+4)
GetPageRegular/From_root-10         757ns ± 1%     706ns ± 1%   -6.75%  (p=0.029 n=4+4)
GetPageRegular/Page_relative-10     838ns ± 1%     786ns ± 1%   -6.16%  (p=0.029 n=4+4)

name                             old alloc/op   new alloc/op   delta
GetPage-10                           312B ± 0%       24B ± 0%  -92.31%  (p=0.029 n=4+4)
GetPageRegular/From_root-10          328B ± 0%      200B ± 0%  -39.02%  (p=0.029 n=4+4)
GetPageRegular/Page_relative-10      360B ± 0%      232B ± 0%  -35.56%  (p=0.029 n=4+4)

name                             old allocs/op  new allocs/op  delta
GetPage-10                           8.00 ± 0%      2.00 ± 0%  -75.00%  (p=0.029 n=4+4)
GetPageRegular/From_root-10          7.00 ± 0%      5.00 ± 0%  -28.57%  (p=0.029 n=4+4)
GetPageRegular/Page_relative-10      9.00 ± 0%      7.00 ± 0%  -22.22%  (p=0.029 n=4+4)
```