codec: fix "prefix of array attribute must be an object name"

[eine]

A change in v0.37 made some of the tests in VUnit fail. #631 fixed some of them, but four are still failing:

tests/acceptance/test_external_run_scripts.py::TestExternalRunScripts::test_com_vhdl_2008
tests/acceptance/test_external_run_scripts.py::TestExternalRunScripts::test_data_types_vhdl_2002
tests/acceptance/test_external_run_scripts.py::TestExternalRunScripts::test_data_types_vhdl_2008
tests/acceptance/test_external_run_scripts.py::TestExternalRunScripts::test_data_types_vhdl_93

According to ghdl/ghdl#1152, this is something that needs to be fixed in VUnit's codebase.

In this PR, helper functions get_encoded_length, get_decoded_range_right and get_decoded_range_left are added. These replace existing helper vars range_left and range_right that were used for RivieraPRO.

To do:

• Should these helper functions be defined elsewhere?
• Do tests pass in RivieraPRO?