scribble/bnf: use 'roman for all BNF metasyntax
scribble/bnf: use 'roman for all BNF metasyntax

This change solves a long-standing problem with using @nonterm{}
inside @tt{} or other constructors that change the font, where
the font would be different (and uglier) than uses outside of font
changes.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket