Refactor slightly BuiltinFunctionEmbedder and check types

Now the arguments and return values types are checked at construction. One checks if the C++ type corresponds to a pugs type.

Closes #29 (closed)

src/language/modules/BuiltinModule.hpp

@@ -32,7 +32,7 @@ class BuiltinModule : public IModule
     }
     catch (std::invalid_argument& e) {
       std::ostringstream os;
-      os << "while defining user function '" << rang::fgB::yellow << name << rang::fg::reset << "'\n";
+      os << "while defining builtin function '" << rang::fgB::yellow << name << rang::fg::reset << "'\n";
       os << e.what();
       throw UnexpectedError(os.str());
     }