heptagon/examples/extern_C

This example show how to import an external function written in C in an Heptagon program.

- The .epi file contains the signature of the function in Heptagon.
- Only functions with no side effects can be safely imported.
- The imported function must have the same calling convention as generated code (inputs given by value, outputs in a f_out structure).
- The generated code will include "mathext.h", that should define the step function