Make CAPI wrapped pointers movable

[BuonOmo]

Should we add the ability to mark these as movable, like we did in the proj4 extension (https://github.com/rgeo/rgeo-proj4/blob/3b55631538b54fd64d7410dd0e2e10be0280235f/ext/proj4_c_impl/main.c#L103-L119)? I figure this would be a good time to do it.

Originally posted by @keithdoggett in #279 (review)

This should be tested using GC.compact or moving specific objects.

Useful resources:

• https://github.com/rgeo/rgeo-proj4/blob/3b55631538b54fd64d7410dd0e2e10be0280235f/ext/proj4_c_impl/main.c#L103-L119 an example
• Make compaction available for Oj::Doc ohler55/oj#653 another example
• https://www.youtube.com/watch?v=H8iWLoarTZc tenderlove introducing the feature
• https://github.com/tenderlove/heap-utils a script to brag about the result

[BuonOmo]

Working on it at klaxit (https://github.com/klaxit/rgeo/tree/movable-pointers)

[BuonOmo]

Fixed by #284