Change contract CI to be inside this repo

[the-frey]

This one is a note to self to have a think about.

Currently, code is pulled in from tags elsewhere and compiled. It probably makes more sense to just compile elsewhere and bring the wasm binaries straight inside this repo.

Pros:

• everything in one place
• quicker to change when things go wrong and need updating

Cons:

• harder to audit
• no way of seeing direct provenance of binary (might be an issue for e.g. unity prop?)