feat(model_theory/substructures): Facts about substructures (#12258)
feat(model_theory/substructures): Facts about substructures (#12258)

Shows that `closure L s` can be viewed as the set of realizations of terms over `s`.
Bounds the cardinality of `closure L s` by the cardinality of the type of terms.
Characterizes `closure L[[A]] s`.



Co-authored-by: Aaron Anderson <6a708ae528ec3e8bbcd87cbd53c0d4f4bab63d64@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib