openFile tweaks

Recent minor revisions to openFile include:

• debut of the close function
• the write function now returns the number of bytes it wrote; keeping track of where you are in a file yourself will almost certainly be a lot faster than repeatedly polling the byteOffset property
• exceptions which occur as a result of an attempt to read a file not opened for reading or write a file not opened for writing will be phrased a bit more intuitively

Next, I think I will tackle read, which will involve some non-trivial buffering logic due to character encoding.

I also still need to figure out a way to reliably trigger an openFile object to be collected as garbage so I can reliably test the callback function which destroys the corresponding C++ object. I found a way. It seems it's not enough to let a script fall though the last curly-brace and then collect the garbage. However, replacing one newly created instance of openFile with another newly created instance seems to make the first "eligible" for garbage collection. I'm not sure why V8 thinks there is a difference once the script has ended, but at least I have a way to test my weak reference callbacks.