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.

test/field/default_to_null.wren

@@ -2,4 +2,4 @@ class Foo {
   write { io.write(_field) }
 }
 
-Foo.new.write // expect: null
+(new Foo).write // expect: null

test/field/multiple.wren

@@ -15,7 +15,7 @@ class Foo {
   }
 }
 
-var foo = Foo.new
+var foo = new Foo
 foo.set(1, 2, 3, 4, 5)
 foo.write
 // expect: 1

test/field/object_reference.wren

@@ -19,15 +19,15 @@ class Node {
   }
 }
 
-var a = Node.new
+var a = new Node
 a.set(null, "a", null)
-var b = Node.new
+var b = new Node
 b.set(null, "b", null)
-var c = Node.new
+var c = new Node
 c.set(a, "c", b)
 a = null
 b = null
-var d = Node.new
+var d = new Node
 d.set(c, "d", null)
 c = null
 d.write

test/field/use_before_set.wren

@@ -3,7 +3,7 @@ class Foo {
   init { _field = "value" }  // ...before an assignment to it.
 }
 
-var foo = Foo.new
+var foo = new Foo
 // But invoke them in the right order.
 foo.init
 foo.write // expect: value