multi: add lnutils to host fundamental utility functions

We also move the `fn/stream.go` into the package `lnutils`. Eventually
we will put all the [utility
functions](https://github.com/lightninglabs/taro/tree/main/chanutils)
into this package.

fn/stream.go

@@ -1,13 +0,0 @@
-package fn
-
-// Map takes an input slice, and applies the function f to each element,
-// yielding a new slice.
-func Map[T1, T2 any](s []T1, f func(T1) T2) []T2 {
-	r := make([]T2, len(s))
-
-	for i, v := range s {
-		r[i] = f(v)
-	}
-
-	return r
-}