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_in_expression_block.wren

@@ -0,0 +1,5 @@
+new Fn { IO.print("ok") // expect error
+}.call // expect error
+
+// The second error is cascaded here. If it starts failing, just remove that
+// expectation.