Handle toString not returning a string in IO.print and IO.write.

Fix #67.

test/io/print_bad_to_string.wren

@@ -0,0 +1,5 @@
+class BadToString {
+  toString { 3 }
+}
+
+IO.print(new BadToString) // expect: [invalid toString]

test/io/write_bad_to_string.wren

@@ -0,0 +1,6 @@
+class BadToString {
+  toString { 3 }
+}
+
+IO.write(new BadToString) // expect: [invalid toString]
+IO.print