Use a binary format for save-analysis

[Xanewok]

Originally created by @nrc (rust-dev-tools/rls-analysis#116):

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 the 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.