Reduce serialised vm size (large bootimage size)

For RPySOM interpreter, the boot image (executable) generated by Zebu is 175mb while the executable from the C backend is less than 1 mb. The main reason is that the Zebu's boot image contains a serialised vm (via Rust's serialisation), and I wasn't careful about what should be serialised so that basically everything is included. The serialised VM probably contribute to 99% of the boot image size.

I estimate if I am being careful about what should be serialised (only what will be used at runtime is worth serialising. #18 is part of the issue), the size can be reduced by at least 5-10 times.

And I am not sure how much this may contribute to the big performance slowdown.