Add ability to call functions requiring arithmetic value conversions

- Conversions are only attempted on a dispatch
 - Conversions are only attempted after a normal dispatch has failed
 - Conversions are only attempted if exactly one function matches
   the signature of the parameters passed in - excluding the mismatched
   arithmetic parameters
 - This feature should not be relied on in performance critical code
   overhead is added for each function call that requires a conversion
   to execute, see the tests performed above.

unittests/arithmetic_conversions_test.cpp

@@ -0,0 +1,58 @@
+// Tests to make sure that type conversions happen only when they should
+
+#include <chaiscript/chaiscript.hpp>
+
+void f1(int)
+{
+}
+
+void f4(std::string)
+{
+}
+
+void f2(int)
+{
+}
+
+void f3(double)
+{
+}
+
+
+int main()
+{
+  chaiscript::ChaiScript chai;
+
+  chai.add(chaiscript::fun(&f1), "f1");
+  chai.add(chaiscript::fun(&f2), "f2");
+  chai.add(chaiscript::fun(&f3), "f2");
+  chai.add(chaiscript::fun(&f1), "f3");
+  chai.add(chaiscript::fun(&f4), "f3");
+
+  // no overloads
+  chai.eval("f1(0)");
+  chai.eval("f1(0l)");
+  chai.eval("f1(0ul)");
+  chai.eval("f1(0ll)");
+  chai.eval("f1(0ull)");
+  chai.eval("f1(0.0)");
+  chai.eval("f1(0.0f)");
+  chai.eval("f1(0.0l)");
+
+  // expected overloads
+  chai.eval("f2(1)");
+  chai.eval("f2(1.0)");
+
+  // 1 non-arithmetic overload
+  chai.eval("f2(1.0)");
+
+  // this is the one call we expect to fail
+  try {
+    chai.eval("f2(1.0l)");
+  } catch (const std::exception &) {
+    return EXIT_SUCCESS;
+  }
+
+  // if the last one did not throw, we failed
+  return EXIT_FAILURE;
+}