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/method/arity.wren

@@ -18,7 +18,7 @@ class Foo {
   method(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) { return a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p}
 }
 
-var foo = Foo.new
+var foo = new Foo
 io.write(foo.method) // expect: 0
 io.write(foo.method(1)) // expect: 1
 io.write(foo.method(1, 2)) // expect: 3

test/method/long_name.wren

@@ -4,4 +4,4 @@ class Foo {
   }
 }
 
-io.write(Foo.new.thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result
+io.write((new Foo).thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result

test/method/operators.wren

@@ -15,7 +15,7 @@ class Foo {
   - { return "prefix -" }
 }
 
-var foo = Foo.new
+var foo = new Foo
 io.write(foo + "a") // expect: infix + a
 io.write(foo - "a") // expect: infix - a
 io.write(foo * "a") // expect: infix * a

test/method/static.wren

@@ -6,8 +6,7 @@ class Foo {
   static bar(arg) { return "on metaclass " + arg }
 }
 
-io.write("on metaclass " + "arg") // expect: on metaclass arg
-io.write(Foo.new.bar)        // expect: on instance
-io.write(Foo.bar)            // expect: on metaclass
-io.write(Foo.new.bar("arg")) // expect: on instance arg
-io.write(Foo.bar("arg"))     // expect: on metaclass arg
+io.write((new Foo).bar)        // expect: on instance
+io.write(Foo.bar)              // expect: on metaclass
+io.write((new Foo).bar("arg")) // expect: on instance arg
+io.write(Foo.bar("arg"))       // expect: on metaclass arg