Fix encoding for mapping with local members (#1152)

Fixes an issue where local members are included in
the mapping entry count.

Also: avoid re-computing Mapping.length

pkl-server/src/test/files/SnippetTests/input/map.pkl

@@ -13,3 +13,9 @@ res4: Mapping = new {
     ["bar"] = 2
   }
 }
+// https://github.com/apple/pkl/issues/1151
+res5: Mapping = new {
+  local self = this
+  ["foo"] = new Dynamic { name = "foo" }
+  ["bar"] = new Dynamic { name = self["foo"].name + "bar" }
+}

pkl-server/src/test/files/SnippetTests/output/map.yaml

@@ -41,4 +41,28 @@
         : 
           - 3
           - 
-            bar: 2
+            bar: 2
+  - 
+    - 16
+    - res5
+    - 
+      - 3
+      - 
+        foo: 
+          - 1
+          - Dynamic
+          - pkl:base
+          - 
+            - 
+              - 16
+              - name
+              - foo
+        bar: 
+          - 1
+          - Dynamic
+          - pkl:base
+          - 
+            - 
+              - 16
+              - name
+              - foobar