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

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.

test/function/no_newline_before_close.wren

@@ -0,0 +1,2 @@
+new Fn {
+  IO.print("ok") }.call // expect error

test/function/syntax.wren

@@ -6,11 +6,7 @@ new Fn {
   IO.print("ok") // expect: ok
 }.call
 
-// No trailing newline.
-new Fn {
-  IO.print("ok") }.call // expect: ok
-
-// Multiple expressions.
+// Multiple statements.
 new Fn {
   IO.print("1") // expect: 1
   IO.print("2") // expect: 2