fn: add slice utilities

[ProofOfKeags]

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.

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[lightninglabs-deploy]

@roasbeef: review reminder @yyforyongyu: review reminder

[Roasbeef]

Linter seems to be failing for some reason:

HEAD detached at pull/8483/merge
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   fn/go.mod
make: *** [Makefile:295: tidy-module-check] Error 1
	modified:   fn/go.sum

Do you get the same locally?

[ProofOfKeags]

I don't get the same thing locally at all. Everything completes just fine.

[guggero]

I don't get the same thing locally at all. Everything completes just fine.

I get the following diff when I run go mod tidy in the fn directory (with Go v1.21.4):

diff --git a/fn/go.mod b/fn/go.mod
index 4d1528efe..e14cb555a 100644
--- a/fn/go.mod
+++ b/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
+)
diff --git a/fn/go.sum b/fn/go.sum
index d066b9820..73345fe5e 100644
--- a/fn/go.sum
+++ b/fn/go.sum
@@ -1,8 +1,21 @@
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/lightninglabs/neutrino/cache v1.1.2 h1:C9DY/DAPaPxbFC+xNNEI/z1SJY9GS3shmlu5hIQ798g=
 github.com/lightninglabs/neutrino/cache v1.1.2/go.mod h1:XJNcgdOw1LQnanGjw8Vj44CvguYA25IMKjWFZczwZuo=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
 github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
+github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
 golang.org/x/exp v0.0.0-20231226003508-02704c960a9b h1:kLiC65FbiHWFAOu+lxwNPujcsl8VYyTYYEZnsOO1WK4=
 golang.org/x/exp v0.0.0-20231226003508-02704c960a9b/go.mod h1:iRJReGqOEeBhDZGkGbynYwcHlctCvnjTYIamk7uXpHI=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

[ProofOfKeags]

I have go 1.21.6 and go mod tidy gives empty output with exit code 0. I'm on a Mac M2 if that has any bearing on this.

EDIT: JK I didn't see that running go mod tidy created a diff in go.sum files

[ProofOfKeags]

Looks to be fixed now.