Strengthen `big_sepL_submseteq` and `big_sep{M,S,MS}_subseteq`
Strengthen big_sepL_submseteq
and big_sep{M,S,MS}_subseteq
to only require the predicate to be affine, instead of the whole BI.
Strengthen big_sepL_submseteq
and big_sep{M,S,MS}_subseteq
to only require the predicate to be affine, instead of the whole BI.