fn: add new fn (func utils) package for generic helper funcs

We start with a simple Map function that can be useful for transforming
objects on the fly.

We may want to eventually make this Taro pakage into a module so we can
just have everything in one place:
https://github.com/lightninglabs/taro/tree/main/chanutils.

fn/stream.go

@@ -0,0 +1,13 @@
+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
+}