acc: add a parser for string -> acc_code

* src/tgba/acc.cc, src/tgba/acc.hh (parse_acc_code): New function.
* src/tgbatest/acc.cc, src/tgbatest/acc.test: Test it.

src/tgba/acc.hh

@@ -968,6 +968,25 @@ namespace spot
     bool uses_fin_acceptance_ = false;
   };
 
+  /// \brief Parse a string into an acc_code
+  ///
+  /// The string should follow the following grammar:
+  ///
+  /// <pre>
+  ///   acc ::= "t"
+  ///         | "f"
+  ///         | "Inf" "(" num ")"
+  ///         | "Fin" "(" num ")"
+  ///         | "(" acc ")"
+  ///         | acc "&" acc
+  ///         | acc "|" acc
+  /// </pre>
+  ///
+  /// Where num is an integer and "&" has priority over "|".  Note that
+  /// "Fin(!x)" and "Inf(!x)" are not supported by this parser.
+  ///
+  /// A spot::parse_error is thrown on syntax error.
+  SPOT_API acc_cond::acc_code parse_acc_code(const char* input);
 }
 
 namespace std