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/if/truth.wren

@@ -1,7 +1,7 @@
 class Iter {
   new(value) { _value = value }
-  iterate(iterator) { return _value }
-  iteratorValue(iterator) { return "value" }
+  iterate(iterator) { _value }
+  iteratorValue(iterator) { "value" }
 }
 
 // False and null are false.