Improve FFI documentation

[SeanTAllen]

[aturley]

I added an issue (#83) for wanting a section about callbacks, but maybe that should be in here?

[SeanTAllen]

I think that is reasonable for now

[0joshuaolson1]

These resources talk about, among other things, how to statically and dynamically link C libraries.

[mnp]

Two more for this list

• FFI functions raising errors talks about how Pony code should declare and handle errors from C but doesn't talk about how the C side should raise an error. We're guessing this is via pony_error() with notes in ponyc/src/libponyrt/pony.h and some examples in ponyc/src/libponyrt/lang/time.c, ponyc/src/libponyrt/lang/posix_except.c, and ponyc/src/libponyrt/lang/socket.c.

• Linking to C Libraries doesn't mention C++. Maybe another section? I've had some success with this method, happy to discuss. ** Write a C wrapper to manage C++ object storage and method invocations. This wrapper would FFI to Pony as normal. ** Make sure the wrapper functions which need to be seen in Pony FFI are declared extern "C" { ... }. ** When linking with GNU tools, be sure to use ponyc --linker g++ because the C++ mangled names and also because of the runtime environment is needed AND be sure to have libstdc++ on your system