python: implement formula.__format__

Fixes #105.

* src/bin/common_trans.cc (quote_shell_string): Move ...
* src/misc/escape.cc, src/misc/escape.hh (quote_shell_string):
... here.
* wrap/python/spot_impl.i: Wrap escape.hh.
* wrap/python/spot.py: Implement formula.__format__.
* wrap/python/tests/ltlsimple.py: Test it.
* NEWS, doc/org/tut01.org, wrap/python/tests/formulas.ipynb: Document
it.

src/misc/escape.cc

@@ -27,6 +27,7 @@
 #include <functional>
 #include <cctype>
 #include <locale>
+#include <cstring>
 #include "escape.hh"
 
 namespace spot
@@ -143,4 +144,35 @@ namespace spot
 	      std::find_if(str.begin(), str.end(),
 			   std::not1(std::ptr_fun<int, int>(std::isspace))));
   }
+
+  std::ostream&
+  quote_shell_string(std::ostream& os, const char* str)
+  {
+    // Single quotes are best, unless the string to quote contains one.
+    if (!strchr(str, '\''))
+      {
+	os << '\'' << str << '\'';
+      }
+    else
+      {
+	// In double quotes we have to escape $ ` " or \.
+	os << '"';
+	while (*str)
+	  switch (*str)
+	    {
+	    case '$':
+	    case '`':
+	    case '"':
+	    case '\\':
+	      os << '\\';
+	      // fall through
+	    default:
+	      os << *str++;
+	      break;
+	    }
+	os << '"';
+      }
+    return os;
+  }
+
 }

src/misc/escape.hh

@@ -65,5 +65,13 @@ namespace spot
   /// \brief Remove spaces at the front and back of \a str.
   SPOT_API void
   trim(std::string& str);
+
+  /// \brief Output \a str between simple quote or double quotes
+  ///
+  /// Simple quotes are preferred unless \a str contains some simple
+  /// quotes.  In that case we use double quotes and escape anything
+  /// that needs to be escaped.
+  SPOT_API std::ostream&
+  quote_shell_string(std::ostream& os, const char* str);
   /// @}
 }