Split up the Nix interop

[thufschmitt]

Separate the bit about calling Nickel (importNickel), and the one really about Organist (importOrganist).

Doesn't change anything semantically, but

1. Makes the flow (hopefully) clearer
2. Allow exporting a lower-level importNickel function that just calls Nickel and imports back the result without the Organist magic

Depends on #203

[yannham]

I think there's an importNcl left somewhere in lib/lib.nix within a comment; a search and replace might get rid of those.

Otherwise, I know it's provided as it is, but maybe a bit more documentation on what importNickel is doing so that non-organist users can make something out of it could be nice. But doesn't have to be in this PR, even if it can.

[thufschmitt]

I think there's an importNcl left somewhere in lib/lib.nix within a comment; a search and replace might get rid of those.

Indeed, a quick grep found a few extras in the documentation, thanks.

Otherwise, I know it's provided as it is, but maybe a bit more documentation on what importNickel is doing so that non-organist users can make something out of it could be nice. But doesn't have to be in this PR, even if it can.

That's fair. I've added a quick blurb to the function. I don't think it deserves much right now (it's still a bit coupled with Organist, so not producion-ready as an independent thing).