Add new Concat utility and use it to fix slice.append gotcha

[phillip-stephens]

While debugging an issue with Phillip/ipv6, I realized that
newSlice := append(inputSlice1, inputSlice2...)
can modify inputSlice1.

While Concat was added in go 1.22, I think that's a bit too cutting edge to mandate people have.

I could not find a linter to automatically check for these sorts of issues, so we'll just have to be careful.

Changes

• added new utility Concat copied from Go's std. lib.
• added testing to show it doesn't modify underlying array
• Used everywhere we were using append to create a new slice