Adding general checkpointing infra to checkpoint inside a pass

[jkalloor3]

These changes allow you to checkpoint subdirectories and make simple changes to checkpoint inside a long pass (such as ScanningGateRemoval on wide blocks). The changes include:

A new RestartCheckpointPass:

• This pass allows you to restart a workflow from a checkpoint. It is more flexible than RestoreIntermediatePass as it allows for sub-blocks to be loaded and performs default passes if a checkpoint does not exist, allowing you to not have to write 2 workflows.

I also add changes to move the checkpoint loading to the ForEachBlockPass to load blocks rather than from the IntermediatePasses themselves. I keep RestoreIntermediate and SaveIntermediate for backwards compatibility, but they should not be needed anymore.