feat: Add StaticSizeCircuit

[lmondada]

I am starting work on an MVP for Badger v2. This will not be merged into main for a while, as i) the performance of portmatching is currently significantly worse than the last version ii) the set of operations and types of circuits that is supported is very limited.

Description of StaticSizeCircuit

This simple struct captures circuits with no classical data (and hence also no parametric angle). This will be the starting point to build patterns and compute updateable hashes.

This could in principle be extended in the future to handle more cases, but neither Agustin nor I are convinced that this should ever find its way to main... Handling more general cases will require work into fast and updateable hashing.

[lmondada]

I'm reworking the definition of StaticSizeCirc, I'll let you know when this is ready to review!