Per-output reference and closure size checks

In structured-attributes derivations, you can now specify per-output
checks such as:

  outputChecks."out" = {
    # The closure of 'out' must not be larger than 256 MiB.
    maxClosureSize = 256 * 1024 * 1024;

    # It must not refer to C compiler or to the 'dev' output.
    disallowedRequisites = [ stdenv.cc "dev" ];
  };

  outputChecks."dev" = {
    # The 'dev' output must not be larger than 128 KiB.
    maxSize = 128 * 1024;
  };

Also fixed a bug in allowedRequisites that caused it to ignore
self-references.