fix clippy lint (#143)

* refactor: Metadata now uses custom deserializer

serde_yaml 0.9.7 resulted in a regression,
accepting `metadata:` (empty metadata mappings)
instead of returning Err.

See dtolnay/serde-yaml#304 and dtolnay/serde-yaml#312

This change uses a custom deserializer to error
in this case.

* fix: fix clippy::unnecessary_to_owned lint

src/specification.rs

@@ -2532,7 +2532,7 @@ impl Graph {
         let mut rv = DemeMap::default();
 
         for deme in &self.demes {
-            if rv.contains_key(&deme.name().to_string()) {
+            if rv.contains_key(deme.name().deref()) {
                 return Err(DemesError::DemeError(format!(
                     "duplicate deme name: {}",
                     deme.name(),