Don't allow calling the root fiber.

The VM used to not detect this case. It meant you could get into a
situation where another fiber's caller had completed. Then, when it
tried to resume that fiber, the VM would crash because there was nothing
to resume to.

This is part of thinking through all the cases around re-entrancy. Added
some notes for that too.

test/core/fiber/call_root.wren

@@ -0,0 +1,8 @@
+var root = Fiber.current
+System.print("begin root") // expect: begin root
+
+Fiber.new {
+  System.print("in new fiber") // expect: in new fiber
+  root.call() // expect runtime error: Cannot call root fiber.
+  System.print("called root")
+}.transfer()