perf(header): optimize when inserting a new type header

src/header/internals/cell.rs

@@ -58,6 +58,11 @@ impl<V: ?Sized + Any + 'static> PtrMapCell<V> {
         PtrMapCell(UnsafeCell::new(PtrMap::Empty))
     }
 
+    #[inline]
+    pub fn with_one(key: TypeId, val: Box<V>)  -> PtrMapCell<V> {
+        PtrMapCell(UnsafeCell::new(PtrMap::One(key, val)))
+    }
+
     #[inline]
     pub fn get(&self, key: TypeId) -> Option<&V> {
         let map = unsafe { &*self.0.get() };

src/header/internals/item.rs

@@ -23,12 +23,11 @@ impl Item {
     }
 
     #[inline]
-    pub fn new_typed(ty: Box<Header + Send + Sync>) -> Item {
-        let map = PtrMapCell::new();
-        unsafe { map.insert((*ty).get_type(), ty); }
+
+    pub fn new_typed<H: Header>(val: H) -> Item {
         Item {
             raw: OptCell::new(None),
-            typed: map,
+            typed: PtrMapCell::with_one(TypeId::of::<H>(), Box::new(val)),
         }
     }