gh-897: stored functions in C/C++

Implement basic support for stored functions in C/C++.

- split away struct func from struct func_def (function
object and function definition object).
- extend box.schema.func.create() to accept 'language' argument.
- move module API related cmake magic to its own file
- add test/lib to gitignore
- update to the new test-run
- add a test