Publish Rust crates (bindings, metaboard) to crates.io

[thedavidmeister]

Raindex consumes this repo's Rust crates via submodule path-deps:

```toml
rain-metadata = { path = "lib/rain.interpreter/lib/rain.metadata/crates/cli" } # alpha exists, want stable
rain-metadata-bindings = { path = "lib/rain.interpreter/lib/rain.metadata/crates/bindings" }
rain-metaboard-subgraph = { path = "lib/rain.interpreter/lib/rain.metadata/crates/metaboard" }
```

`rain-metadata` is on crates.io at `0.0.2-alpha.6` — needs a stable release. The other two crates aren't published at all.

Once published, raindex (rainlanguage/raindex#2586) will switch `Cargo.toml` to crates.io version pins.