Set up visibility controls for public symbols in the library

[WardBrian]

This PR makes the necessary changes to set the default symbol visibility to "hidden". It's worth reading more about this on the GCC Wiki.

The summary:

• By default, every symbol in a shared library is available to the linker
• If you are explicit about which symbols actually should be public, your compiler can then perform optimizations based on the assumption that the remaining code doesn't need to be accessible
• The Microsoft compilers do this by default, with the added wrinkle of also distinguishing between "public and provided by the current translation unit" (dllexport) vs "public and provided by someone else" (dllimport)

[WardBrian]

What is the use case of BRIDGESTAN_EXPORT? It doesn't seem like we use it, or maybe I just missed that.

It gets defined on line 23 of the makefile. The purpose is such that when we're building a shared library on Windows, the functions are marked as dllexport, but if somebody else writes #include <bridgestan.h>, they get a dllimport-marked function instead.

Should this have some doc about it?

It could, but I'm not sure what to write or where it would be best suited. Is the idea we'd be documenting it for end users or for developers/ourselves?

size measurements

Binary size (using model ode_sundials):

Optization setting	Size in bytes, before	Size in bytes, after
O3	1,108,800	1,016,888
O2	1,065,152	969,376
Os	1,041,056	860,928

Number of symbols (nm -C -D ode_sundials_model.so | wc -l). These are both at O3, it mattered much less for this in terms of total numbers:

Before	After
1559	956

Note: I picked this model as sort of a worst-case, since the sundials symbols are exported. For something like bernoulli, the "after" is ~500 symbols.

speed

This is harder since we don't have a ton of precise benchmarks. At a basic level, things like loading the library will get faster (less to load into memory, and less symbols to scan over to find our functions), but testing to see if this genuinely provides an increased opportunity for optimizations is difficult

[roualdes]

Re BRIDGESTAN_EXPORT. Thanks. That makes sense.

This seems great. Thanks again for finding this and thanks for incorporating it into BridgeStan.