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/return/return_null_if_brace.wren

@@ -1,6 +0,0 @@
-var f = new Fn {
-  if (true) { return }
-  IO.print("bad")
-}
-
-IO.print(f.call) // expect: null