More file IO!

Can now create, delete, and write to files.

test/io/file/write_bytes_after_close.wren

@@ -0,0 +1,10 @@
+import "io" for File
+
+var file = File.create("file.temp")
+file.close()
+
+System.print(Fiber.new {
+  file.writeBytes("one")
+}.try()) // expect: File is not open.
+
+File.delete("file.temp")