Validate Runnable API FFI version

[cohix]

When a Wasm module is loaded into Reactr, it should be checked for compatibility. This should be done via an FFI function that returns the version of the API it was built against.

This method will acheive two things, firstly it will "prove" that the module is a Runnable, and second it will allow Reactr to check that it is a compatible version (the method of determining compatibility will need to be explored as part of this issue)

[danielledeleo]

Yes please :)