zlib: revert back to Functions

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

PR-URL: #13374
Fixes: #13358
Ref: #13370
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>

Author: jasnell