Add v1 file format for MappedParallelComputationGraph and from_v1 conversion functions

[elliottslaughter]

Adds two things:

• V1MappedParallelComputationGraph
• from_v1 conversion from v1 back to normal objects

Originally written with Claude, but the code got almost entirely ripped out and rewritten so essentially everything is done by hand at this point. Some of the v1 object representations also changed to make serialization possible or more convenient.

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This change is 

[elliottslaughter]

Copying the PR feedback for visibility:

Yeah, that part is pretty messy and difficult to read, and definitely not ready for merge. All the casts should to be ommitted in favor of adding the corresponding code into nonnegative_int (or at the very least pulled out into a separate lambda), a bunch of the loops should be changed to use code from utils/containers, and all of the emplaces are a bit unidiomatic for this codebase. Also, it would probably be cleaner to structure it as function from V1KwargDataflowGraph to KwargDataflowGraph, and then add the label handling on top as a separate function.

I refactored the casts and emplace, currently working through the container code and V1KwargDataflowGraph split.