get rid of bitrotted Show backend

was going to be too much work to fix it up for changes to Backend class,
and it only ever half-worked anyway.  If we ever want to revive it, it's
there in the git history.
get rid of nullPrim

implement strut and phantom the right way: using leafU
Merge pull request #171 from diagrams/units-simple-backend

Updates to work with `Backend` redesign