handle optional layout in template conversion

This is a partial fix for http://yellowgrass.org/issue/SDF/73. 

To fix this completely, `symbol-to-elem` needs to handle all symbols. Currently, it cannot handle `-CF` or `-LEX` variants of EBNF operators applied to `LAYOUT`.

I am also not sure if `LAYOUT?-CF` should yield the same as `LAYOUT-CF` in the conversion, as proposed by this fix.