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
[auto] Merge from Ruler

    • #2116882 — Add `hoa ruler:assert`. (Ivan Enderlin, 2013-07-10)

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Merge remote branch 'library-ruler/master'