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/super/call_other_method.wren

@@ -11,7 +11,7 @@ class Derived is Base {
   }
 }
 
-Derived.new.bar
+(new Derived).bar
 // expect: Derived.bar
 // expect: Base.foo
 

test/super/call_same_method.wren

@@ -11,6 +11,6 @@ class Derived is Base {
   }
 }
 
-Derived.new.foo
+(new Derived).foo
 // expect: Derived.foo
 // expect: Base.foo

test/super/indirectly_inherited.wren

@@ -13,6 +13,6 @@ class C is B {
   }
 }
 
-C.new.foo
+(new C).foo
 // expect: C.foo
 // expect: A.foo