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/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