Don't panic in AssetPath deserialization

[greeble-dev]

Objective

AssetPath deserialization panics if the path is malformed. This seems a bit rude, and prevents the user from getting deserialization errors that would help track down the problem.

Solution

Instead of calling AssetPath::parse, call AssetPath::try_parse and handle any errors.

Now, calling ron::de::from_str on "a/b.test#" returns a useful error:

Err(SpannedError {
    code: Message(
        "Asset label must be at least one character. Either specify the label after the '#' or remove the '#'",
    ),
    span: Span {
        start: Position { line: 1, col: 2, },
        end: Position { line: 1, col: 12 },
    },
})

The PR also removes the visit_string method of the AssetPath deserializer. visit_string is an optimization that avoids copies when the deserializer can take ownership of the string (see serde docs). But AssetPath didn't take ownership of the string, so there's no reason to implement it.

Testing

cargo test -p bevy_asset
cargo run --example world_serialization