This adds support for `$check` cells in chformal and adds a `-lower` mode which converts `$check` cells into `$assert` etc. cells with a `$print` cell to output the `$check` message.
This adds support for `$check` cells in chformal and adds a `-lower` mode which converts `$check` cells into `$assert` etc. cells with a `$print` cell to output the `$check` message.