Optimize stringbuffer_append to avoid strlen calls

[Algunenano]

Related to ​https://trac.osgeo.org/postgis/ticket/4543

stringbuffer_append is used to output txt (ST_AsText), KML and X3D, and it calculates the length of the string parameter even when the library already knows it (constant or the output of a previous function call).

I plan to add an extra function (stringbuffer_append_len) that allows you to pass the length of the string and simply memcpy's it.

[Paul Ramsey <pramsey@…>]

In 5304a6f6/git:

Optimize stringbuffer_append to avoid strlen calls, References #4614

[Raúl Marín <git@…>]

In 87922037/git:

Optimize stringbuffer_append to avoid strlen calls

References #4614

[Algunenano]

With both commits, I'm seeing a 5-6% improvement in ST_AsText (670.234 ms -> 637.996 ms)