Optimization: execute const object bodies and typechecks only once (#915)

If the object member is const, it only needs to be executed once, and all children in the prototype chain can use its cached value.

There is a possible other optimization here, not included in this PR: we can avoid adding these values to the child object's cachedMembers.

pkl-core/src/test/files/LanguageSnippetTests/input/basic/constModifier5.pkl

@@ -0,0 +1,13 @@
+// should result in only one trace per property
+class MyClass {
+  // property body only gets executed once
+  const prop = trace("hello")
+
+  // typecheck only happens once
+  const prop2: Int(let (_ = trace("hello again")) true) = 1
+
+  res: Int
+}
+
+a = new MyClass { res = 1 }
+b = (a) { res = 2 }

pkl-core/src/test/files/LanguageSnippetTests/output/basic/constModifier5.err

@@ -0,0 +1,12 @@
+a {
+  prop = "hello"
+  prop2 = 1
+  res = 1
+}
+b {
+  prop = "hello"
+  prop2 = 1
+  res = 2
+}
+pkl: TRACE: "hello" = "hello" (file:///$snippetsDir/input/basic/constModifier5.pkl)
+pkl: TRACE: "hello again" = "hello again" (file:///$snippetsDir/input/basic/constModifier5.pkl)