Add `hoa ruler:assert`.

Examples:

    $ hoa ruler:assert -x=2 -y=6 'x in (1, 2, 4) and x < y'
    $ echo $?
    0

    $ hoa ruler:assert -x=4 -y=1 'x in (1, 2, 4) and x < y'
    $ echo $?
    1