Much improved filament path - no escape!

Further improved dbgeorge's design:
► added upper guides to stop wiggly filament getting around the gear
► shrunk the hole around the gear (also to stop filament getting there)
► extruded a nozzle around the PTFE tube entry to hold it more steadily (half of it sticks in)
► removed the hole below the spring - improves structure if printed with high infill density

Note: the added upper guides fit around the original gear, I haven't tested with replacement gears

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