More robust metadata serialization

[NicolasHug]

To serialize the container/stream metadata to go from C++ to Python, we manually create a JSON string in the C++ code and parse that in Python to get a dict.

We should try to find a more robust way to do that. There are a few alternatives, but ultimately we are limited to what custom ops allow us to return. Some potential alternatives:

• use a proper JSON library (in C++)
• pack all the info into tensors.
• If we're OK to drop the compile/export support for metadata, we could just bind the C++ structs to Python with pybind11.