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/README.md

@@ -0,0 +1,4 @@
+# `lnutils`
+
+This package provides fundamental types and utility functions that can be used
+by all other packages.