fix(frontends/lean): bug in scope construct

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean): make sure pretty print and parse test passes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/kernel): add more theorems useful for simplification

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