refactor(frontends/lean): rename action::is_compatible to action::is_equal

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): allow user to provide a terminator for 'foldr' and 'foldl'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean): remove keywords 'class' and '[class]'

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(build): add 'clean-olean' custom-target

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