util: Move copy3 in utils with types, to avoid code duplication

[bew]

Extracted from #1396

[bew]

Well test failed with no space left on device 😅

But all functional tests have passed

======== 536 tests from 34 test files ran. (17958.35 ms total)
PASSED   536 tests.
[100%] Built target functionaltest

[L3MON4D3]

Well test failed with no space left on device 😅

Oh dang, I guess that's because we have three versions of nixpkgs in there o.O
I'll have to look into setting up a cache for these tests sometime, or do a gc between runs

[L3MON4D3]

It passes locally (for all versions) 👍 :D