Normalize metadata into metadata per instance and metadata per path (#107)

* Begin the metadata merge trek

* Tidy up path metadata, entry API, begin implementing

* Flesh out use of PathMap Entry API

* Metadata per instance is a go

* Tidy up naming for metadata per instance

* SnapshotMetadata -> SnapshotContext

server/src/path_map.rs

@@ -1,4 +1,5 @@
 use std::{
+    collections::hash_map,
     path::{self, Path, PathBuf},
     collections::{HashMap, HashSet},
 };
@@ -31,6 +32,16 @@ impl<T> PathMap<T> {
         self.nodes.get(path).map(|v| &v.value)
     }
 
+    pub fn get_mut(&mut self, path: &Path) -> Option<&mut T> {
+        self.nodes.get_mut(path).map(|v| &mut v.value)
+    }
+
+    pub fn entry<'a>(&'a mut self, path: PathBuf) -> Entry<'a, T> {
+        Entry {
+            internal: self.nodes.entry(path),
+        }
+    }
+
     pub fn insert(&mut self, path: PathBuf, value: T) {
         if let Some(parent_path) = path.parent() {
             if let Some(parent) = self.nodes.get_mut(parent_path) {
@@ -105,4 +116,28 @@ impl<T> PathMap<T> {
 
         current_path
     }
+}
+
+pub struct Entry<'a, T> {
+    internal: hash_map::Entry<'a, PathBuf, PathMapNode<T>>,
+}
+
+impl<'a, T> Entry<'a, T> {
+    pub fn or_insert(self, value: T) -> &'a mut T {
+        &mut self.internal.or_insert(PathMapNode {
+            value,
+            children: HashSet::new(),
+        }).value
+    }
+}
+
+impl<'a, T> Entry<'a, T>
+    where T: Default
+{
+    pub fn or_default(self) -> &'a mut T {
+        &mut self.internal.or_insert(PathMapNode {
+            value: Default::default(),
+            children: HashSet::new(),
+        }).value
+    }
 }