Add DEFGATE AS CIRCUIT

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It Would Be Nice to define gates by some circuit decomposition. This is different than DEFCIRCUIT since those are simply for expansion and not meant to represent "Gates".

DEFGATE CZ p q AS CIRCUIT:
    H p
    CNOT p q
    H p

This is not a DEFCIRCUIT! This is the expression of a unitary operation (which is not expanded!) as a sequence of gates.

Concrete qubit indices shouldn't be allowed.

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(I'm open to another keyword besides CIRCUIT.)

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Other ideas: AS PRODUCT, AS SEQUENCE, AS OTHER-GATES

Other note: If somebody implements this, you'd need to check for circularity.