Derivation "ATerm" file format

For historical reasons, derivations are stored on-disk in ATerm format.

Derivations are serialised in one of the following formats:

• Derive(...)

For all stable derivations.

• DrvWithVersion(<version-string>, ...)

The only version-strings that are in use today are for experimental features:

• "xp-dyn-drv" for the dynamic-derivations experimental feature.