reduce SpatialObject to 40 bytes in-memory

Did not improve performance

benchstat bench_master_sep4/bench_CockroachDB.txt bench_master_sep4_spatialproto/bench_CockroachDB.txt
name                                   old ms     new ms     delta
Test/nyc/nyc-intersects                75.7 ±15%  75.2 ±21%    ~     (p=0.101 n=47+50)
Test/nyc/nyc-dwithin                    219 ± 7%   217 ± 9%    ~     (p=0.143 n=49+47)
Test/nyc/nyc-coveredby                 77.3 ±14%  76.6 ±17%    ~     (p=0.202 n=47+47)
Test/nyc/nyc-expand-blocks             87.0 ± 8%  86.7 ± 9%    ~     (p=0.510 n=48+46)
Test/nyc/nyc-dwithin-and-dfullywithin   192 ± 6%   187 ± 7%  -2.43%  (p=0.000 n=49+47)
Test/postgis_geometry_tutorial/11.2b   1.63 ±32%  1.60 ±30%    ~     (p=0.440 n=49+48)
Test/postgis_geometry_tutorial/11.6    1.67 ±23%  1.71 ±19%    ~     (p=0.159 n=46+47)
Test/postgis_geometry_tutorial/12.1.2  0.89 ±26%  0.90 ±29%    ~     (p=0.513 n=46+47)
Test/postgis_geometry_tutorial/12.2.3  1.34 ±22%  1.33 ±13%    ~     (p=0.959 n=45+46)
Test/postgis_geometry_tutorial/12.2.4  1.27 ±27%  1.32 ±19%    ~     (p=0.102 n=48+48)
Test/postgis_geometry_tutorial/13.0    1.51 ±27%  1.61 ±23%  +6.49%  (p=0.008 n=49+48)
Test/postgis_geometry_tutorial/13.1a    230 ± 7%   226 ± 8%  -1.79%  (p=0.001 n=48+49)
Test/postgis_geometry_tutorial/13.1c   24.2 ±13%  23.8 ±12%    ~     (p=0.245 n=49+48)
Test/postgis_geometry_tutorial/13.2     304 ± 9%   303 ± 7%    ~     (p=0.937 n=48+46)
Test/postgis_geometry_tutorial/14.1a   1.66 ±28%  1.58 ±22%  -5.04%  (p=0.020 n=47+49)
Test/postgis_geometry_tutorial/14.2b   5.93 ±21%  5.91 ±19%    ~     (p=0.807 n=48+50)
Test/postgis_geometry_tutorial/14.2c   3.61 ± 8%  3.59 ± 9%    ~     (p=0.474 n=47+48)
Test/postgis_geometry_tutorial/14.3c   31.1 ± 8%  30.7 ± 6%    ~     (p=0.063 n=50+50)
Test/postgis_geometry_tutorial/15.0    1.50 ±16%  1.53 ±16%    ~     (p=0.213 n=50+49)

Release justification:
Release note (<category, see below>): <what> <show> <why>