test: replace a stray string literal with a Nonterminal constant.
parse+read: make r5js.parse.Nonterminal typedef an actual class.
parse: implement r5js.parse.OneNonterminal_#desugar.
parse: grammar: replace one-element seqs with the one element where possible.
parse: simplify some r5js.parse.bnf.Rule ctors.
parse: split out a generic DesugarableRule interface from r5js.parse.bnf.Rule.