fix(frontends/lean): '@' explicit mark

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/max_sharing): take binder annotations into account

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/module): make sure decls from imported modules have module_idx > 0

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): bug in process_flex_rigid, also cleanup the code and break it into different cases

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