Implement support for constant keys in our Mapping type

We implement handling of constant keys (keys prefixed with
`KeyPrefix::Constant`) directly in the Mapping implementation. Keys with
other prefixes are inserted into the map unchanged.

To make the implementation more streamlined (and potentially faster), we
switch the type of `const_keys` to be a `HashSet` instead of a `Vec`,
which gives us `O(1)` removal of elements (for `Mapping::remove()`).

To insure integrity of the constant key tracking, We remove the
`map_as_mut()` method on the Mapping implementation which would allow
callers to break the constant key invariants that the other functions
provide.

Some functions (`insert`, `get_mut`, and `entry`) will return an Error
value when called for a key that's marked constant.

src/node/mod.rs

@@ -98,8 +98,9 @@ impl Node {
 
         // Convert serde_yaml::Mapping into our own Mapping type
         let mut params: Mapping = n._params.clone().into();
-        // Inject `_reclass_` meta parameter into parameters
-        params.insert("_reclass_".into(), n.meta.as_reclass().into());
+        // Inject `_reclass_` meta parameter into parameters, return an error if someone has marked
+        // `_reclass_` or a subkey of it as constant.
+        params.insert("_reclass_".into(), n.meta.as_reclass().into())?;
         n.parameters = params;
 
         Ok(n)

src/node/nodeinfo.rs

@@ -45,18 +45,24 @@ impl NodeInfoMeta {
 
     /// Generates a Mapping suitable to use as meta parameter `_reclass_`
     pub(crate) fn as_reclass(&self) -> Mapping {
-        let mut namedata = Mapping::new();
-        namedata.insert("full".into(), self.name.clone().into());
-        namedata.insert(
-            "parts".into(),
-            Value::Sequence(vec![self.name.clone().into()]),
-        );
-        namedata.insert("path".into(), self.name.clone().into());
-        namedata.insert("short".into(), self.name.clone().into());
+        let namedata: Vec<(Value, Value)> = vec![
+            ("full".into(), self.name.clone().into()),
+            (
+                "parts".into(),
+                Value::Sequence(vec![self.name.clone().into()]),
+            ),
+            ("path".into(), self.name.clone().into()),
+            ("short".into(), self.name.clone().into()),
+        ];
+        let namedata = Mapping::from_iter(namedata);
 
         let mut pmeta = Mapping::new();
-        pmeta.insert("environment".into(), self.environment.clone().into());
-        pmeta.insert("name".into(), Value::Mapping(namedata));
+        pmeta
+            .insert("environment".into(), self.environment.clone().into())
+            .unwrap();
+        pmeta
+            .insert("name".into(), Value::Mapping(namedata))
+            .unwrap();
 
         pmeta
     }