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/implicit_receiver/locals_shadow_getter.wren

@@ -1,5 +1,5 @@
 class Foo {
-  bar { return "getter" }
+  bar { "getter" }
 
   test {
     IO.print(bar) // expect: getter