Python: Introduce gdb.RecordGap class

As discussed here: https://sourceware.org/ml/gdb-patches/2017-04/msg00157.html

A gap is not an instruction and it should not pretend to be one.
gdb.Record.instruction_history is now a list of gdb.RecordInstruction and
gdb.RecordGap objects.  This allows the user to deal with Gaps in the record
in a more sane way.

gdb/python/py-record.h

@@ -37,4 +37,8 @@ typedef struct
   enum record_method method;
 } recpy_record_object;
 
+/* Create a new gdb.RecordGap object.  */
+extern PyObject *recpy_gap_new (int reason_code, const char *reason_string,
+				Py_ssize_t number);
+
 #endif /* GDB_PY_RECORD_H */