Compact extruder with symmetric mount and fan support

First version of Prusa nozzle used an asymmetrical aluminium mount as cooling plate. Newer version uses a symmetrical one, that means compact extruder part needs a change. This is my version. Rest of the parts may be obtained from https://github.com/josefprusa/Prusa3-vanilla/tree/master/distribution

Please note I have upgraded this part to the new x-carriage using 30mm of distance between support holes.

Thicker MK7-like drive gears won't fit in the original extruder. I have changed the channel for the drive gear so now it should be possible to use MK7 drive gear (untested).

New version added, now with holes for a 40mm fan and a duct.

New version just added with bearing capture to avoid it from jumping outside of its place.