c: paper over missing functions in the shared libraries

This is in part to work around that most installed .so files aren't compatible with snegg yet, we can probably drop this later.

During startup, we load every function we know about from the .so file - some of which may not exist with the current version of the installed library. Replace those with a function that raises a NotImplementedError so we can get past the import stage - if the program never uses that function then it doesn't matter anyway.