Bump to 3.1.1

Remove trailing slash from directory root paths.

Nix doesn't parse those properly for some reason

Properly propagate changes to nix-buffer-directory-name

Make nix-buffer-directory-name customizable

Add nix-buffer-after-load-hook.

Fixes #3

Allow dir-locals.nix to not specify any drvs

Run the build asynchronously

Minor cleanups

Rename local copy of f-traverse-upwards

Reference find-file-hook in commentary