Supporting more equation environments

[tmke8]

I think I need to introduce something like this:

#[derive(Debug, Default)]
struct SequenceState {
    class: Class,
    table_state: Option<TableState>,
}

struct TableState {
    num_rows: usize, // needed for `multline`
    max_num_col_separators: usize, // needed for `alignedat` (and for `gather` and `multline` to ensure it's 0)
}

if table_state is None, we can introduce the check from #189 as well