Benchmark runners

Each directory should have an executable ./build and ./run.
The runners should use the following environment variables:
- MHB_ITERATIONS: times to run each input.
- MHB_INPUT_DIR: path to directory containing inputs. Files should be read from this directory and used as the inputs.
- MHB_HTML_ONLY: if set to 1, minify_css and minify_js should be disabled.
The output should be a JSON array of tuples, where each tuples contains the input name, output size, iterations, and execution time in seconds (as a floating point value).
- The execution time should be measured using high-precision monotonic system clocks where possible.