In the year of our lord 2024 we should not be writing for loops
for standard operations. Here we introduce named slice operations
not found in the golang slices package. Note all these functions
are pure.
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.
