zlib: revert back to Functions

Using ES6 Classes broke userland code. Revert back to functions.

PR-URL: https://github.com/nodejs/node/pull/13374
Fixes: https://github.com/nodejs/node/issues/13358
Ref: https://github.com/nodejs/node/pull/13370


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>