* Greatly reduced the amount of stack space used by the Nix expression

  evaluator.  This was important because the NixOS expressions started
  to hit 2 MB default stack size on Linux.

  GCC is really dumb about stack space: it just adds up all the local
  variables and temporaries of every scope into one huge stack frame.
  This is really bad for deeply recursive functions.  For instance,
  every `throw Error(format("error message"))' causes a format object
  of a few hundred bytes to be allocated on the stack.  As a result,
  every recursive call to evalExpr2() consumed 4680 bytes.  By
  splitting evalExpr2() and by moving the exception-throwing code out
  of the main functions, evalExpr2() now only consumes 40 bytes.
  Similar for evalExpr().

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