Replace reflect.SliceHeader with unsafe.Slice

[shane-ns1]

Fixes part of #20

This PR fixes the go vet warning caused by the use of reflect.SliceHeader() by replacing it with unsafe.Slice().

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I'm not 100% this actually works as desired, as the unsafe and reflect libraries both feel like voodoo to me. There don't seem to be tests in this area, but maybe I missed them?

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This requires a version bump for Go:

./main.go:246:14: unsafe.Slice requires go1.17 or later (-lang was set to go1.15; check go.mod)

This is probably reasonable, since Go supports two major releases at a time:
https://go.dev/doc/devel/release#policy

And we're on Go 1.21, meaning that anything older than Go 1.19 is unsupported.

OTOH, I can see how dropping support for language release from less than 3 years ago could seem a bit aggressive. ¯\_(ツ)_/¯

[wojas]

I agree that the bump is reasonable. We loosely follow the Go support policy, as following those minor releases is a good idea and generally safe. We currently only test in CI with 1.19+. Older versions of Go may or may not work.