Switch to hidden symbol visibility as default

[pmatilai]

C++ symbol visibility is a different beast from that of C as I found out in #3679. We need to move to hidden visibility as default, and explicit exports for exporting symbols to the ABI.

We should've really done that a long ago already, but that got mixed up with symbol versioning (#1127) which is a more intrusive change and requires a soname bump to go with it. Changing symbol visibility doesn't, as long as the exported symbols just stay the same.

This doesn't affect the desire to introduce symbol versioning later on.