Clean up WithMeta handling

[kylewlacy]

The WithMeta type is used to track the stack trace a recipe was created in TypeScript: since recipe evaluation is lazy, it can be really hard to track this down from just the recipes themselves. This metadata isn't used at all in Brioche today, but the eventual goal is to give better error messages (e.g. when a build fails, we could point to the line in TypeScript where the error occurred)

Turns out, this metadata wasn't properly being deserialized when going from TypeScript to Rust, so it always came back empty. This PR fixes that, and makes a few changes so the WithMeta<_> wrapper type is a bit simpler:

• Update serde::Deserialize impl on WithMeta<_> to try and deserialize the meta field (serialization still skips it). This also means the type that WithMeta wraps must now deserialize from an object
• Remove WithMeta for directory entries in the Directory type. Since these are string values (representing recipe hashes), this no longer worked with the deserialization changes to WithMeta