Make PropertyMetadataImpl a data class

To allow property delegates to use property metadata as a key in the hash map,
and to improve debugging experience

compiler/testData/codegen/box/delegatedProperty/propertyMetadataEqualsHashCodeToString.kt

@@ -0,0 +1,36 @@
+import java.util.HashSet
+
+class A {
+    val foo: String by O
+    val Int.foo: String by O
+
+    fun foo42() = 42.foo
+}
+
+val foo: String by O
+val Int.foo: String by O
+
+object O {
+    val metadatas = HashSet<PropertyMetadata>()
+
+    fun get(t: Any?, p: PropertyMetadata): String {
+        metadatas.add(p)
+        return ""
+    }
+}
+
+fun box(): String {
+    A().foo
+    A().foo42()
+    foo
+    42.foo
+
+    if (O.metadatas.size() != 1)
+        return "Too many different PropertyMetadata instances: ${O.metadatas}"
+
+    val m = O.metadatas.iterator().next()
+    if (m.toString() != "PropertyMetadata(name=foo)")
+        return "Wrong toString(): $m"
+
+    return "OK"
+}