fix(library/unifier): missing test in flex_rigid

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(frontends/lean/elaborator): remove dead code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fet(frontends/lean): allow coercions to sort-class in the types of variable and definitions

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