fn: create new sub-module

In this commit, we make a new sub-module from the `fn` package. This
lets other projects use it w/o pulling in all of lnd, and also other
sub-packages within lnd.

We also add a temp replace directive for new module.  We'll remove this
once we tag the final version.

fn/go.mod

@@ -0,0 +1,3 @@
+module github.com/lightningnetwork/lnd/fn
+
+go 1.19