process: improve performance of nextTick

This replaces TickObject with an object literal. This offers
performance improvements of up to ~20%.

PR-URL: https://github.com/nodejs/node/pull/8932


Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>