Optimise Rune compilation times

[Michael-F-Bryan]

We are seeing issues in hammerd where the time it takes for rune build to complete is becoming prohibitive.

Possible ways to improve compilation are:

• Pre-fetch all common dependencies - it seems like the time required to download dependencies, especially our proc blocks (git dependencies require fetching the entire repo) is a bit of a bottleneck
• Pre-compile common dependencies - build the dependencies inside the Docker image then just copy them (maybe use hard links to avoid actual copying) into the directory used to build a Rune
• Aggressively remove dependencies from proc blocks and runic-types - this feels like an easy win because we pull in serde-derive and serde-json. We could always manually write some code that generates JSON strings
• Use some sort of shared build cache like sccache