perf improvement: generate derivations ourselves, bypassing nix evaluation

[lf-]

I was talking to @puckipedia about this project and we were both somewhat surprised that Nix evaluations are invoked for each command invocation via nix-build on some Nix source, rather than emitting .drv files directly and skipping the nix language.

Emitting .drv files directly, then calling nix-store --add and nix-store --realise would save time and avoid the overhead of O(modules) number of Nix evaluations (although I do not have traces of exactly how much).

[ocharles]

This was always a plan of mine, but I skipped it because of the (minor) complexity of working out how to get a ghc derivation to depend on. We could probably do that with a single (constant) nix-instantiate though.