Use a binary format for save-analysis

[nrc]

Serialisation/deserialisation takes a non-trivial amount of time. If we are sure that the versions match (because of using Rustup) we could just dump he save-analysis data to disk in a binary format. That should mean smaller downloads plus faster analysis processing.

This will require effort both here and in the compiler (and its build system/distribution), but should be transparent to clients.