feat(library/aliases): add 'exceptions' and support for universes to add_aliases procedure, add for_each_universe method to environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/scoped_map): add 'keep' method for closing a scope without undoing operations

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(emacs): update keywords

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/parser): add namespace/section/end commands, add support for explicit universe levels, fix Type notation'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/builtin_cmds): add 'variable' command family

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