this means fennelrepl in nix-shell will prefer local
source files to generated lua files, making it easier
to change library code without restarting the shell
This requires adding LFS as a dependency because native Lua has
no way to iterate a directory, but it seems to be Not Huge and
hopefully we'll have other uses for it
Runs fennel using a Lua compiled with the same options as the
host system, and with packages set up so it can find all the local
Lua packages
To shorten the dev feedback loop further, allows FENNEL_PATH to be set
on the command line so you can point directly it at the Fennel sources
for some library you're working against instead of having to run
nix-build and compile them to Lua
Daniel Barlow