fix: cannot call SetPrimaryCore after using a Tee logger

[moul]

Previous behavior

Calling SetPrimaryCore after being configured with a zapcore.Tee causes a panic: runtime error: comparing uncomparable type zapcore.multiCore

$ go test -v ./... -count 1 -run TestTeeCore
=== RUN   TestTeeCore
failed to IncreaseLevel: invalid increase level, as level "fatal" is allowed by increased level, but not by existing core
--- FAIL: TestTeeCore (0.00s)
panic: runtime error: comparing uncomparable type zapcore.multiCore [recovered]
        panic: runtime error: comparing uncomparable type zapcore.multiCore

goroutine 19 [running]:
testing.tRunner.func1.2(0x612ea0, 0xc000099060)
        /nix/store/mjpyjqs53d77c691c46kdd97a4094pjm-go-1.16.7/share/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000082900)
        /nix/store/mjpyjqs53d77c691c46kdd97a4094pjm-go-1.16.7/share/go/src/testing/testing.go:1146 +0x4b6
panic(0x612ea0, 0xc000099060)
        /nix/store/mjpyjqs53d77c691c46kdd97a4094pjm-go-1.16.7/share/go/src/runtime/panic.go:965 +0x1b9
github.com/ipfs/go-log/v2.(*lockedMultiCore).ReplaceCore(0x7d7600, 0x699640, 0xc0000ba138, 0x699680, 0x805650)
        /home/moul/go/src/github.com/ipfs/go-log/core.go:97 +0xab
github.com/ipfs/go-log/v2.setPrimaryCore(0x699680, 0x805650)
        /home/moul/go/src/github.com/ipfs/go-log/setup.go:161 +0x65
github.com/ipfs/go-log/v2.SetPrimaryCore(0x699680, 0x805650)
        /home/moul/go/src/github.com/ipfs/go-log/setup.go:156 +0x73
github.com/ipfs/go-log/v2.TestTeeCore(0xc000082900)
        /home/moul/go/src/github.com/ipfs/go-log/setup_test.go:258 +0x2b1
testing.tRunner(0xc000082900, 0x662380)
        /nix/store/mjpyjqs53d77c691c46kdd97a4094pjm-go-1.16.7/share/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
        /nix/store/mjpyjqs53d77c691c46kdd97a4094pjm-go-1.16.7/share/go/src/testing/testing.go:1238 +0x2b3
FAIL    github.com/ipfs/go-log/v2       0.008s
FAIL

New behavior

No panic

h/t @gfanton

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[BigLep]

@MDobak : can you please take a look here as this looks related to your recent changes?

[Stebalien]

This is unfortunate... but I can't think of a better fix. In the "good" case where pointers are being used, this will be equivalent to ==.

Could you fix the comparison in DeleteCore as well?

[moul]

@Stebalien, done

[Stebalien]

Thanks!