src/lstate.js: Implement freestack(), call from lua_close
src/lstate.js: Implement freestack(), call from lua_close

Work towards https://github.com/giann/fengari/commit/354d659f577fc27969784400c8c1e6090756da7b#commitcomment-21975897

fengari-lua/fengari,dlbd/fengari,giann/fengari,daurnimator/fengari