update to use slugs.convert_to_slugsin
rewrite bitfield conversion in slugs interface

- bug fix: dict item order is not dependable (using "first" can yield incorrect results). Use list instead.
- the LSB bit name is known, searching not needed
- rename the functions to reflect their purpose
- add docstrings
- pep8: snake case var names
- use short names for local vars
- use str.format for readability
- iterate over bits, math not needed
create variable dict once only
reduce nesting
bugfix: introduced in 5ed12b3673b571c2afe3462316f3345ebd629441
rm trailing whitespace

and replace the `,\s*` possibly following the LSB by `,\s` for testing purposes.
improve error checking
handle both integers as str and int as admissible values
add test for slugs interface

that imports jtlvint_test.basic_test and reuses it.