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/this/closure.wren

@@ -1,9 +1,6 @@
 class Foo {
-  getClosure {
-    return new Fn { toString }
-  }
-
-  toString { return "Foo" }
+  getClosure { new Fn { toString } }
+  toString { "Foo" }
 }
 
 var closure = (new Foo).getClosure

test/this/nested_class.wren

@@ -9,14 +9,14 @@ class Outer {
         method {
           IO.print(this) // expect: Inner
         }
-        toString { return "Inner" }
+        toString { "Inner" }
       }
 
       (new Inner).method
     }.call
   }
 
-  toString { return "Outer" }
+  toString { "Outer" }
 }
 
 (new Outer).method

test/this/nested_closure.wren

@@ -1,15 +1,6 @@
 class Foo {
-  getClosure {
-    return new Fn {
-      return new Fn {
-        return new Fn {
-          return toString
-        }
-      }
-    }
-  }
-
-  toString { return "Foo" }
+  getClosure { new Fn { new Fn { new Fn { toString } } } }
+  toString { "Foo" }
 }
 
 var closure = (new Foo).getClosure

test/this/this_in_method.wren

@@ -1,6 +1,6 @@
 class Foo {
-  bar { return this }
-  baz { return "baz" }
+  bar { this }
+  baz { "baz" }
 }
 
 IO.print((new Foo).bar.baz) // expect: baz

test/this/this_in_static_method.wren

@@ -4,7 +4,7 @@ class Foo {
     IO.print(this.bar) // expect: bar
   }
 
-  static bar { return "bar" }
+  static bar { "bar" }
 }
 
 Foo.test