Detect stack overflows

Previously, if the Nix evaluator gets a stack overflow due to a deep
or infinite recursion in the Nix expression, the user gets an
unhelpful message ("Segmentation fault") that doesn't indicate that
the problem is in the user's code rather than Nix itself.  Now it
prints:

  error: stack overflow (possible infinite recursion)

This only works on x86_64-linux and i686-linux.

Fixes #35.

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