Add several new functions to the C API:
StorePath operations:
- nix_store_path_hash: Extract the hash part from a store path
- nix_store_create_from_parts: Construct a store path from hash and name
Derivation operations:
- nix_derivation_clone: Clone a derivation
- nix_derivation_to_json: Serialize a derivation to JSON
Store operations:
- nix_store_drv_from_store_path: Load a derivation from a store path
Test the new functions, and improve documentation of some existing
functions to better distinguish them, also.
Co-authored-by: Tristan Ross <tristan.ross@determinate.systems>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
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