Micro optimizations to the serialization process

[Algunenano]

I spent a few hours having a look at Expanded Object Headers but found it a too intrusive change for the benefit it would give us.

On the other hand, there are some small improvements that can be made to the serialization - deserialization process to make it slightly faster, which isn't too impresive but since I have alredy done it I might as well push it and make it available.

PR in ​https://github.com/postgis/postgis/pull/577

[Raúl Marín <git@…>]

In 3d049c8/git:

Micro optimizations to the serialization process

Closes #4746
Closes ​https://github.com/postgis/postgis/pull/577