perf: avoid two-step json deser

Author: ss2165

hugr-core/src/envelope/reader.rs

@@ -144,12 +144,10 @@ impl<R: BufRead> EnvelopeReader<R> {
     /// Returns package and the combined extension registry
     /// of the provided registry and the package extensions.
     fn decode_json(&mut self) -> Result<Package, PackageEncodingError> {
-        let val: serde_json::Value = serde_json::from_reader(&mut self.reader)?;
-
         let super::package_json::PackageDeser {
             modules,
             extensions: pkg_extensions,
-        } = serde_json::from_value(val.clone())?;
+        } = serde_json::from_reader(&mut self.reader)?;
         let modules = modules.into_iter().map(|h| h.0).collect_vec();
         let pkg_extensions = ExtensionRegistry::new_with_extension_resolution(
             pkg_extensions,