feat(build): add tests/lean/slow test directory, and add nat_wo_hints.lean file that elaborates nat.lean without using any hint

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lean): cleanup add_delayed_theorem method

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lean): create theorem_queue class

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