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/grouping.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar = value { return value }
+  bar = value { value }
 }
 
 var foo = new Foo