This is a major change!  Almost all the components have changed to
smaller versions, with 0603 and 0402 passives and a QFN microcontroller.
The board now has four layers, with internal layers used for VBUS and
GND.  The board now measures a mere 25×30 mm!  The SWD header and Boot
switch footprints have been moved to the bottom to help with this
shrinkage.  Two test points were added to the bottom as well to make it
possible to power the board without using the USB connector.  The
mounting holes are now slightly smaller (better sized for M3), and only
17 mm apart instead of the old 18 mm.

To sum up, everything is smaller and that's almost entirely a good
thing, except now it's not really possible to build one by hand.

It's a USB Type-C connector just like the one used in v0.1 and v0.2, but
with only SMT pins (except the fixing pins, of course).  They're
slightly cheaper and probably nicer for automated assembly, but
impossible to hand-solder.  That's okay though: where we're going, we
don't need hand-solderability.

Apparently `git commit -a` doesn't add untracked files.  Oops.

It was so close to the pad that it wasn't on the soldermask, causing the
fab to remove it almost entirely.  This should fix that problem.

There was previously no way of seeing where the business end of the USB
connector was.  To solve this, I added a crude outline of the connector
on the F.Fab layer.

Tested to probably not blow anything up if assembled properly!