Need to bound the memory use of Jsonnet evaluation

[sparkprime]

The interpreter currently knows how many objects are live, but not how large they are. If it had this extra information, it would be possible to place a bound on the amount of memory that can be used during execution. This makes it safer to run Jsonnet in a server environment.

[sparkprime]

The most reliable way to do this is, e.g. when evaluating Jsonnet server side, is to fork a new process to run Jsonnet (e.g. just run the standard commandline binary) and then ulimit that process.