Unify body handling.

Blocks, functions, and methods now have the same code for handling
their bodies.

This means that single-line methods work like single-line functions:
they return the result of their expression.

test/constructor/default.wren

@@ -1,5 +1,5 @@
 class Foo {
-  toString { return "Foo" }
+  toString { "Foo" }
 }
 
 // Classes inherit the argument-less "new" one by default.

test/constructor/infix_class_expression.wren

@@ -1,5 +1,5 @@
 class Foo {
-  + other { return "Foo " + other }
+  + other { "Foo " + other }
 }
 
 IO.print(new Foo + "value") // expect: Foo value

test/constructor/new.wren

@@ -3,7 +3,7 @@ class Foo {
   new(a) { IO.print(a) }
   new(a, b) { IO.print(a + b) }
 
-  toString { return "Foo" }
+  toString { "Foo" }
 }
 
 // Can overload by arity.

test/constructor/superclass.wren

@@ -4,7 +4,7 @@ class A {
     _field = arg
   }
 
-  aField { return _field }
+  aField { _field }
 }
 
 class B is A {
@@ -14,7 +14,7 @@ class B is A {
     _field = arg1
   }
 
-  bField { return _field }
+  bField { _field }
 }
 
 class C is B {
@@ -24,7 +24,7 @@ class C is B {
     _field = "c"
   }
 
-  cField { return _field }
+  cField { _field }
 }
 
 var c = new C