Make `allUnits` work.

[goldfirere]

Right now,

import Data.Units.SI
import Data.Units.US

do units <- allUnits
   makeQuasiQuoter "u" [] units

fails with a non-letter somewhere. There are multiple problems.

1. Even if I restrict the imports, Template Haskell finds everything that's exported from any module, regardless of whether it's actually in scope. This may not be fixable.
2. I'm pretty sure it's Ohms or Angstroms that are causing the trouble. But maybe not, because both symbols respond True to isLetter. In any case, it's not working.

I have no idea if the problem is in this package, in units-parser or in units-defs.