Apply memory limits to individual buckets

[Chris Warburton]

We can run MLSpec without a memory cutoff, but it's dangerous as it'll
gladly eat up all of our RAM.

Instead, we apply a cutoff using withTimeout. That works, but it's a
little bit crappy since the whole point of using buckets is that we get
to use multiple processes. Hence we should allow our limits to be set on
the individual runners, rather than the invoker.

One way to do this would be to augment `wrapper.sh` in nix-eval, e.g. to
accept a new env var like `NIX_EVAL_WRAP_PROGRAM`. If found, that would
be invoked with the given command (`runhaskell ...`) as an argument.

We can then set it to a script which calls `wrapProgram`. Note that we'd
probably have to propagate any given limit through "manually", since it
may be running in a different environment to our invoking CLI.

Note that MLSpec is used by hashspec as well, so this should apply to
both.