clk2fflogic: Always correctly handle simultaneously changing signals

This is a complete rewrite of the FF replacing code.

The previous implementation tried to implement the negative hold time by
wrapping async control signals individually with pulse stretching. This
did not correctly model the interaction between different simultaneously
changing inputs (e.g. a falling ALOAD together with a changing AD would
load the changed AD instead of the value AD had when ALOAD was high; a
falling CLR could mask a raising SET for one cycle; etc.).

The new approach first has the logic for all updates using only sampled
values followed by the logic for all updates using only current values.
That way, e.g., a falling ALOAD will load the sampled AD value but a
still active ALOAD will load the current AD value.

The new code also has deterministic behavior for the initial state: no
operation is active when that operation would depend on a specific
previous signal value. This also means clk2fflogic will no longer
generate any additional uninitialized FFs.

I also documented the negative hold time behavior in the help message,
copying the relevant part from async2sync's help messages.

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