Performance of lwgeom_cmp, avoid memcpy

[pramsey]

The memcpy involved in moving the bits from the float4 coordinate representation to a int representation can be avoided with a type-punning union.

[pramsey]

Solution in ​https://github.com/postgis/postgis/pull/144

[pramsey]

In 15874:

Use type-pun to avoid memcpy in building morton key (References #3871)

[pramsey]

In 15875:

Use type-pun to avoid memcpy in building morton key (Closes #3871)
Fix from Komzpa at ​https://github.com/postgis/postgis/pull/144

[pramsey]

In 15876:

Fix union issue (References #3871)

[pramsey]

In 15877:

Fix union issue (References #3871)