process: slightly simplify next tick execution

Get rid of separate function to call callback from _tickCallback as
it no longer yields worthwhile performance improvement.

Move some code from nextTick & internalNextTick into TickObject
constructor to minimize duplication.

PR-URL: https://github.com/nodejs/node/pull/16888


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>