Small spoolholder remix

So I liked the idea of JohnSL's spool holder, but I didn't have any M8 nuts and bolts lying about, so remixed it for use without nuts and bolts.

Pretty snug depending on your calibration. You'll probably need to tidy things up a bit with a scalpel. Shouldn't need support.

For one filament spool, print:
- 2x spool_bearingholder.stl
- 8x spool_bearinggrip.stl

Alternatively, just print:
2x spool_bearingholder_full

You'll need:
- 4x 608 bearings.