The way we serialise output makes cross compiling across architectures hard

[ltratt]

At the moment we serialise a number of usize things which means if you cross-compile for a different machine word size (e.g. you cross-compile on a 64-bit machine with a WASM target) you end up with deserialisation errors. There are two solutions:

1. The quick hack is to remove the troublesome usize things in the serialisation path.
2. The long term solution is to have a better (almost certainly custom written) serialisation format which can deal with such things.