README preserve example doesnt work
Am I misunderstanding the readme? I can't find a combination of flags that makes something like `a,b = circfill, rectfill` appear in the output:
```lua
--preserve: !circfill, !rectfill
circfill, rectfill = --[[preserve]]circfill, --[[preserve]]rectfill
circfill(10,10,20); circfill(90,90,30)
rectfill(0,0,100,100); rectfill(20,20,40,40)
--preserve: !rnd
rnd = --[[preserve]]rnd
rnd(12)
```
`shrinko8 --minify test.lua - -f lua --no-minify-spaces`
```lua
circfill, rectfill = circfill, rectfill
circfill(10,10,20) circfill(90,90,30)
rectfill(0,0,100,100) rectfill(20,20,40,40)
a = rnd
a(12)
```

---

This doesn't actually affect me, its just a bug(?) I noticed recently.

Notice that it _does_ work for `rnd`, likely because rnd is (correctly) missing from [this list](https://github.com/thisismypassport/shrinko8/blob/3b6ea85853b93a3a932210fa2d0b80744b932478/pico_process.py#L53)?