make benchmarking functions public

[willow-ahrens]

This is required for benchmark loops which would like to test the runtime of assembly. e.g.

  // Assemble output indices and numerically compute the result
  auto time = benchmark(
    [&y]() {
      y.setNeedsAssemble(true);
      y.setNeedsCompute(true);
    },
    [&y]() {
      y.assemble();
      y.compute();
    }
  );