Ingesting lots of data into Nix: builtins.fromCBOR / builtins.fromBSON

[Ericson2314]

Is your feature request related to a problem? Please describe.

Multiple third-party tools are known rely on reading large amounts of data into Nix expressions with builtins.fromJSON. This is a performance consideration, especially for CI use cases. Adding a way to read in structured data in a compact representation would solve that problem for those cases.

Describe the solution you'd like

Allowing ingesting data from a 3rd-party-standardized binary format that is easy to parse very quickly.

Describe alternatives you've considered

#7825, which has stability and expressive power challenges, but on the other hand supports lazy loading and random access.

Additional context
As proposed by @Ericson2314 and @edolstra and discussed by the Nix team.

Priorities

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