Export / import functions to / from a file (#1642)

* export and import functions

* refactor + works for few primitives

* nit

* allow primitives with state

* nit

* nit

* simplify serialize / deserialize

* fix for constants

* python bindings

* maybe fix serialize failure case

* add example

* more primitives, training kind of works

* same result for python and c++

* some fixes

* fix export

* template it up

* some simplificatoin

* rebase

* allow kwargs and multiple functions

* exporter

* more primitives for exporting

* deal with endianness

* handle invalid stream

* add docstring

python/src/convert.cpp

@@ -331,7 +331,7 @@ PyScalarT validate_shape(
         t = pycomplex;
       } else {
         std::ostringstream msg;
-        msg << "Invalid type  " << nb::type_name(l.type()).c_str()
+        msg << "Invalid type " << nb::type_name(l.type()).c_str()
             << " received in array initialization.";
         throw std::invalid_argument(msg.str());
       }