Added an example of how to import external functions

examples/extern_C/README

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+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

examples/extern_C/imports.ept

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+open Mathext
+
+fun f(a:float) returns (o:float)
+let
+  o = mycos(a);
+tel

examples/extern_C/mathext.c

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+#include <math.h>
+#include "mathext.h"
+
+void mycos(float a, mycos_out *out)
+{
+  out->o = cos(a);
+}

examples/extern_C/mathext.epi

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+val fun mycos(a:float) returns (o:float)

examples/extern_C/mathext.h

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+#ifdef MATHEXT_H
+#define MATHEXT_H
+
+struct mycos_out {
+  float o;
+};
+
+void mycos(float a, mycos_out *o);