Reading files!

- File.close()
- File.open()
- File.read()
- file.readBytes()

And a few other little methods. Still lots more work to do, but it's a
start.

test/io/file/close.wren

@@ -0,0 +1,19 @@
+import "io" for File
+import "scheduler" for Scheduler
+
+// See also: is_open.wren.
+
+var file = File.open("test/io/file/close.wren")
+
+System.print(file.close()) // expect: null
+
+// Can call multiple times.
+file.close()
+
+// If already closed, returns synchronously.
+Scheduler.add {
+  System.print("does not print")
+}
+
+file.close()
+System.print("sync") // expect: sync