don't repeatedly look up ast internal symbols

these symbols are used a *lot*, so it makes sense to cache them. this
mostly increases clarity of the code (however clear one may wish to call
the parser desugaring here), but it also provides a small performance
benefit.

src/libexpr/parser-state.hh

@@ -43,6 +43,7 @@ struct ParserState {
     SourcePath basePath;
     PosTable::Origin origin;
     const ref<InputAccessor> rootFS;
+    const Expr::AstSymbols & s;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);
     void dupAttr(Symbol attr, const PosIdx pos, const PosIdx prevPos);