Add support for leading - in character sets
          #347
        
          
      
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This adds support for leading unescaped
-in sets, since that's trivial to do manually without access to desatomat. Adding support for trailing unescaped-without access to desatomat is dramatically harder, so I have no done that.The actual code generated by desatomat will likely differ slightly from what I'm doing manually here, but the result should still be the same.
This is a partial fix for #346. Support for trailing unescaped
-is likely trivial with access to desomat, but I don't appear to have access to that, and the web version times out, so I can't do that bit myself :(