This doesn't fix the motor bug, but it doesn't make it worse. However,
it does simplify the trees quite a bit, so it should be easier to debug.
It seems the problem has something to do with fold_constants messing up
dagged aliases: in particular, const-folding multiplication by e0123 in
3d PGA as fold_constants sees it as 1.
And vis-versa.
I'm not sure what I was thinking, but I've decided that not being able
to cast the pseudo-scalar from float to double (for printf etc) was a
bug.
Simple k-vectors don't use structs for their layout since they're just
an array of scalars, but having the structs for group sets or full
multi-vectors makes the system alignment agnostic.
