Tronxy X1/X3 screwless extruder and part fan holder

Originally planning to replicate the design by [Adverse Effects](https://www.thingiverse.com/Adverse_Effects/designs) because it didn't fit my printer, I kept the base idea of a screwless holder but changed the overall design to something simpler and less bulky.

Since Tronxy seems to have differences between kits, here are the relevant measurements on mine, which I got in November 2017:

* Hot end width: 20 mm
* Hot end height: 10 mm
* Distance between hot end and heat sink: ~7.5 mm

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