Support __toString as a special attr set member

[jwiegley]

If an attr set contains a member __toString, it will be called whenever the attr set is coerced to a string, received the same attr set as its argument. The result is then coerced to a string.

See https://github.com/NixOS/nix/blob/master/src/libexpr/eval.cc#L1538

[jwiegley]

I've added support in Exec.hs, but for now it only applies when coercing for derivation. We need to use this coerceToString function in the same places that nix does.

[layus]

In particular, it needs to apply during conversion:

hnix/src/Nix/Convert.hs

Lines 158 to 160 in a3b95b2

	NVSet' s _ -> case M.lookup "outPath" s of
	Nothing -> pure Nothing
	Just p -> fromValueMay p

But the convert instance does not allow to pass the callFunc utility. Any idea ? Delete the Convert NixString instance ?