fix(library/inductive_unifier_plugin): unification problem failure on problems with inductive datatypes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/elaborator): instantiate assigned metavariables before collecting unassigned ones

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): do not let a unification plugin to 'prioritize' a flex-flex constraint, and add missing case

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>