misc: get rid of small _xc functions

Small library currently depends on Tarantool core through 'exception.h'.
This is not the way to go. Let's drop this dependency and instead of
moving _xc functions to Tarantool repo we can just stop using them. Our
current policy is to panic on OOM in case of runtime allocation.

Part of #7327

NO_DOC=<OOM behaviour is not documented>
NO_CHANGELOG=<no OOM expectations>
NO_TEST=<no test harness for checking OOM>