Feature request: handle memory overflow

[sorawee]

From a student in CS173: If Pyret really exhausts memory, Pyret should stop the program gracefully, not crashing the tab in browser.

[jswrenn]

Chrome, with its proprietary performance.memory API, is basically the only browser we could do this for.

[jpolitz]

Sigh. Yeah. I think we need to wait for browser tech on this one.