refactor: replace Foo.new constructor syntax with new Foo and simplify superclass constructor calls

Remove the `this new` constructor declaration syntax in favor of a `new` keyword that creates instances directly. Superclass constructors are now invoked via bare `super(args)` instead of `super.new(args)`, eliminating the semantic weirdness of calling a constructor on a partially-constructed instance. Update the compiler to treat `new` as a keyword token, replace the `isMethod` flag with `methodName`/`methodLength` fields, and change the VM's `NEW` opcode to pop the class from the stack before creating the instance. Add a default `new` native method on Object that returns `this`. Update all benchmarks and tests to use the new syntax.

benchmark/binary_trees.wren

@@ -1,13 +1,13 @@
 // Ported from the Python version.
 
 class Tree {
-  this new(item, depth) {
+  new(item, depth) {
     _item = item
     if (depth > 0) {
       var item2 = item + item
       depth = depth - 1
-      _left = Tree.new(item2 - 1, depth)
-      _right = Tree.new(item2, depth)
+      _left = new Tree(item2 - 1, depth)
+      _right = new Tree(item2, depth)
     }
   }
 
@@ -27,9 +27,9 @@ var stretchDepth = maxDepth + 1
 var start = OS.clock
 
 io.write("stretch tree of depth " + stretchDepth.toString + " check: " +
-    Tree.new(0, stretchDepth).check.toString)
+    new Tree(0, stretchDepth).check.toString)
 
-var longLivedTree = Tree.new(0, maxDepth)
+var longLivedTree = new Tree(0, maxDepth)
 
 // iterations = 2 ** maxDepth
 var iterations = 1
@@ -44,7 +44,7 @@ while (depth < stretchDepth) {
   var check = 0
   var i = 1
   while (i < iterations + 1) {
-    check = check + Tree.new(i, depth).check + Tree.new(-i, depth).check
+    check = check + new Tree(i, depth).check + new Tree(-i, depth).check
     i = i + 1
   }
 

benchmark/method_call.wren

@@ -1,5 +1,5 @@
 class Toggle {
-  this new(startState) {
+  new(startState) {
     _state = startState
   }
 
@@ -11,7 +11,8 @@ class Toggle {
 }
 
 class NthToggle is Toggle {
-  this new(startState, maxCounter) super.new(startState) {
+  new(startState, maxCounter) {
+    super(startState)
     _countMax = maxCounter
     _count = 0
   }
@@ -31,7 +32,7 @@ var start = OS.clock
 var n = 1000000
 var i = 0
 var val = true
-var toggle = Toggle.new(val)
+var toggle = new Toggle(val)
 
 while (i < n) {
   val = toggle.activate.value
@@ -41,7 +42,7 @@ while (i < n) {
 io.write(toggle.value)
 
 val = true
-var ntoggle = NthToggle.new(val, 3)
+var ntoggle = new NthToggle(val, 3)
 
 i = 0
 while (i < n) {