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.

lnutils/stream.go

@@ -0,0 +1,13 @@
+package lnutils
+
+// 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
+}