Two function names updated to match changes in expl3
Use \__chk_if_free_cs:N in place of \chk_if_free_cs:N

Ideally, this would not be used at all. However, expl3 does not currently
cover the types of information checked here. As this is a patch from me to
a third-party, the minimum change seems appropriate.
Merge pull request #6 from josephwright/master

Changes to track expl3