refactor(frontends/lean): replace '[private]' modifier with 'private
definition' and 'private theorem', '[private]' is not a hint.
refactor(frontends/lean): replace '[protected]' modifier with 'protected definition' and 'protected theorem', '[protected]' is not a hint.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add 'irreducible' as syntax sugar for 'reducible [off]'