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/setter/associativity.wren

@@ -1,6 +1,6 @@
 class Foo {
   new(value) { _value = value }
-  toString { return _value }
+  toString { _value }
   bar = value {
     _value = value
     return value