lint: unused functions
```sh
$ echo 'function f1() return 2 end' | shrinko8 - --lint
$ echo 'local function f2() return 2 end' | shrinko8 - --lint
Lint warnings:
-:1:16: Local 'f2' isn't used
```

The second lint (`local function f2`) works as expected, but I'm surprised that the first one (`function f1`) passes with no complaints. (Maybe this is much harder to detect due to _ENV possibilities..?)

Context: I want to find all unused functions in my cart and remove them, but I can't make them all local because then declaration order causes issues.

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Relatedly, is there any way for me to get a list of all functions that are only referenced one single time? (to find candidates for manual inlining, to save tokens)