Using type size in condition yields exception

[senier]

When using Some_Type'Size in a condition, PyRFLX crashes with an exception when parsing a message. Most likely this problem is similar to #525 and the solution is to substitute occurences of Some_Type'Size by the actual size of the type.

package Test is
   type T1 is mod 2**8;
   type Message is
      message
         F1 : T1
            then F2
               if F1 = Test::T1'Size;
         F2 : T1;
      end message;
end Test;

#!/usr/bin/env python3
from rflx.pyrflx import PyRFLX
SPECS = PyRFLX.from_specs(["test.rflx"], True)
TEST = SPECS["Test"]
message = TEST["Message"]
message.parse(bytes([8, 3]))

Traceback (most recent call last):
  File "./test.py", line 6, in <module>
    message.parse(bytes([8, 3]))
  File "pyrflx/typevalue.py", line 726, in parse
    current_field = self._fields[current_field_name]
KeyError: ''

[treiher]

@rssen Could you please have a look at that? I suppose the fix for this issue should be rather simple, probably _simplified_mapping just has to be extended.