Replace `MGuard` with `MFail`

stdpp/base.v

stdpp/countable.v

stdpp/fin_maps.v

stdpp/gmap.v

stdpp/gmultiset.v

stdpp/hashset.v

stdpp/list.v

stdpp/numbers.v

stdpp/option.v

stdpp/sets.v

stdpp/tactics.v

stdpp/vector.v

stdpp_unstable/bitvector.v

tests/decidable.v