Turn on zlib for larger builds

The docs indicate zlib support:  https://circuitpython.readthedocs.io/en/latest/docs/library/uzlib.html

However, it doesn't look like it's implemented for the nRF52840.  I tried the Feather nRF52840 Express and the PCA10059.

I'm trying to write a CircuitPython Png decoder and this would be very helpful.