Add LTL past operator

[fabianschuiki]

CIRCT has recently gained the circt.ltl.past FIRRTL intrinsic which allows us to emit $past expressions in SystemVerilog assertions. Add a corresponding Sequence.past(...) operator, which allows the user to refer to past values of a sequence, optionally with a cycle delay != 1, and optionally with an explicit clock instead of the one inferred for the sequence.

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[jackkoenig]

Overall looks good, a couple of nits to consider but non-blocking