fn: add slice utilities

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.

fn/go.mod

@@ -4,5 +4,12 @@ go 1.19
 
 require (
 	github.com/lightninglabs/neutrino/cache v1.1.2
+	github.com/stretchr/testify v1.8.1
 	golang.org/x/exp v0.0.0-20231226003508-02704c960a9b
 )
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)