* [runtime] Clean up public symbols. Fixes#5124.
Clean up public symbols, by:
* Symbols that don't need to be public (most of them), can be private.
* Prefix all public symbols with `xamarin_`.
* Add a test to ensure we don't introduce new public symbols.
* Use C symbols instead of mangled C++ symbols, since those are easier to
handle in the test.
This minimizes the chance of getting into a symbol clash with another native library.
Fixes https://github.com/xamarin/xamarin-macios/issues/5124.
* Some test fixes.
Sebastien Pouliot
Rolf Bjarne Kvinge