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/descriptor.wren

@@ -0,0 +1,11 @@
+import "io" for File
+
+var file = File.open("test/io/file/descriptor.wren")
+
+// We can't test for a specific value since it's up to the OS, but it should be
+// a positive number.
+System.print(file.descriptor is Num) // expect: true
+System.print(file.descriptor > 0) // expect: true
+
+file.close()
+System.print(file.descriptor) // expect: -1