Similar to buildGo.nix, the library derivations carry information
about their dependencies which is merged when a load file is
instantiated.
The load files are created when compiling libraries, but will in the
future also be created when wrapping SBCL and dumping images.
Adds a Nix function to build a Lisp library out of a specified set of
Nix files. All files are combined into a single FASL.
This is by design only compatible with SBCL (for now).