s2: Optimise readFloat64 to speed up Rect decoding.
s2: Optimise readFloat64 to speed up Rect decoding.

This speeds up this path by removing allocations.

name           old time/op    new time/op     delta
RectDecode-12     491ns ± 4%      275ns ± 4%  -44.07%   (p=0.000 n=9+10)

name           old alloc/op   new alloc/op    delta
RectDecode-12      112B ± 0%        56B ± 0%  -50.00%  (p=0.000 n=10+10)

name           old allocs/op  new allocs/op   delta
RectDecode-12      9.00 ± 0%       2.00 ± 0%  -77.78%  (p=0.000 n=10+10)

name           old speed      new speed       delta
RectDecode-12  66.8MB/s ± 5%  120.2MB/s ± 4%  +79.86%  (p=0.000 n=10+10)

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

golang/geo