investigate using gnu visibility attributes to improve link times

[ericniebler]

Experiment with tagging all internal implementation details with __attribute__((__hidden__)), or hide the symbols with pragmas. See if link time improves.

[lukeyeh]

hi @ericniebler is this still up for grabs? happy to give it a go.

[ericniebler]

Still up for grabs. I have no idea if this will make any difference, but worth a shot.