[WIP] replace bit_tools.iter_bits with classical_sim.ints_to_bits

[charlesyuan314]

This is just a first attempt at #811 to gather feedback and to see what tests fail.

Known issues:

• iter_bits still exists even though it is entirely redundant.
• iter_bits no longer supports sign.
• iter_bits no longer raises ValueError for too-wide inputs.

[charlesyuan314]

@mpharrigan @tanujkhattar @fdmalone Do you think this is a sane approach?

P.S. Looks like I can't directly request a PR review without being a member of the quantumlib org.

[fdmalone]

I think this makes sense. The bigger migration might be to just replace iter_bits with the corresponding to_bits methods from the register data types(e.g. https://github.com/quantumlib/Qualtran/blob/main/qualtran/_infra/data_types.py#L215) which will handle the signed-ness etc. But maybe out of scope for this PR.

[mpharrigan]

Yeah, since this one of the issues with this consolidation (as currently presented) is that it drops the data-type-specific configuration options (signedness, input validation...) that are now encoded in the QDType implementations (which didn't exist when all these iter_bits functions were first introduced).

Can you find where these methods are used within the library and see if it would be sensible to replace them with e.g. QUInt(...).to_bits(x)? And maybe port these free functions to use the qdtype methods under-the-hood but with a backwards-compatible interface

[charlesyuan314]

Closing as I'm currently working on a PR to replace the bit_tools functions with QDType implementations as @mpharrigan suggested.