Allow %prec to define a token in the grammar.

Previously if `%prec 'x'` was the first mention of the token `x` then
cfgrammar would say "'x' isn't a token" which is clearly incorrect.
This commit is maybe best thought of as a quick fix of sorts: it makes
`%prec 'x'` define a token `x` if it doesn't already exist.

However -- and my memory is completely fuzzy as to whether this is a
Yacc compatibility thing or not -- this means that `%prec 'x'` and
`%prec x' (i.e. with and without quote marks) have the semantic meaning
because `parse_token` treats both interchangeably. This seems consistent
with other parts of the grammar even if a little odd.

cfgrammar/src/lib/yacc/parser.rs

@@ -677,12 +677,11 @@ impl YaccParser {
                 syms.push(Symbol::Token(sym, span));
             } else if let Some(j) = self.lookahead_is("%prec", i) {
                 i = self.parse_ws(j, true)?;
-                let (k, sym, _) = self.parse_token(i)?;
-                if self.ast.tokens.contains(&sym) {
-                    prec = Some(sym);
-                } else {
-                    return Err(self.mk_error(YaccGrammarErrorKind::PrecNotFollowedByToken, i));
+                let (k, sym, span) = self.parse_token(i)?;
+                if self.ast.tokens.insert(sym.clone()) {
+                    self.ast.spans.push(span);
                 }
+                prec = Some(sym);
                 i = k;
             } else if self.lookahead_is("{", i).is_some() {
                 let (j, a) = self.parse_action(i)?;
@@ -1736,17 +1735,6 @@ x"
             src,
         )
         .expect_error_at_line(src, YaccGrammarErrorKind::IllegalString, 3);
-
-        let src = "
-        %%
-        S: 'A' %prec B;
-        B: ;
-        ";
-        parse(
-            YaccKind::Original(YaccOriginalActionKind::GenericParseTree),
-            src,
-        )
-        .expect_error_at_line(src, YaccGrammarErrorKind::PrecNotFollowedByToken, 3);
     }
 
     #[test]