libexpr: quote reserved keys when printing

This fixes a bug in commands like `nix eval' which would emit invalid attribute
sets if they contained reserved keywords such as "assert", "let", etc.

These keywords will not be quoted when printed, making them valid expressions.
All keywords recognized by the lexer are quoted except "or", which does not
require quotation.

src/libexpr/print.hh

@@ -35,6 +35,12 @@ namespace nix {
      */
     std::ostream & printAttributeName(std::ostream & o, std::string_view s);
 
+    /**
+     * Returns `true' is a string is a reserved keyword which requires quotation
+     * when printing attribute set field names.
+     */
+    bool isReservedKeyword(const std::string_view str);
+
     /**
      * Print a string as an identifier in the Nix expression language syntax.
      *