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/size_after_close.wren

@@ -0,0 +1,6 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+file.close()
+
+file.size // expect runtime error: File is not open.