Adds a function that can launch Sly with a pre-configured SBCL for a
Lisp derivation in the depot.
This makes it convenient to spin up development environments for Lisp
libraries and programs by simply calling `M-x nix/sly-from-depot RET
tools.something`.
This relies on `nix-depot-path` being configured currently as I have
not yet reliably added the depot to my NIX_PATH on all machines.
Adds an Emacs library with so far a grand total of one helper function
that can prefetch and insert a git repository at point.
This is very useful for the various Go repo imports I am doing at the
moment.