More file IO!

Can now create, delete, and write to files.

test/io/file/delete_open.wren

@@ -0,0 +1,14 @@
+import "io" for File, Stat
+
+// Create a file and don't close it.
+var file = File.create("file.temp")
+
+File.delete("file.temp")
+
+file.close()
+
+// TODO: More graceful API to tell if a file exists.
+var error = Fiber.new {
+  Stat.path("file.temp")
+}.try()
+System.print(error) // expect: no such file or directory