Together with #21158, this gets us 95% there for lazy desrialization. Implementation is generally straightforward. At some point I wanted to unify the new global state with (parts of) the checker state, and potentially get rid of a bunch of named_type callbacks we pass around, but decided to do this later in a separate PR, since this is not strictly necessary for parallel checking.

Although this PR itself only makes fixup phase (which is the smaller part of deserialization) lazy, it already shows small performance improvements:

• ~net zero on self-check
• few percent faster cold torch with 4 workers
• ~15% faster warm torch

cc @JukkaL