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/function/newline_body.wren

@@ -0,0 +1,4 @@
+var f = new Fn {
+  // Hi.
+}
+IO.print(f.call) // expect: null