Make OpaqueOp::signature non-optional.

[aborgna-q]

Currently it's an optional field as an optimisation for the serial format, where we could avoid encoding some signatures if we know the relevant extension will be available at load-time.

However, the current deserialiser does not support this and just fails.

We should temporarily drop this optimisation, at least until we can pass extension registers to the deserialiser.

This (plus #690) is the root cause of #683.