net/netip for Go < 1.18

This is an extraction of the net/netip package from Go 1.18's standard library, for use in Go 1.17.

Originally extracted by @zx2c4 at golang.zx2c4.com/go118/netip.

When Go 1.18 is released, that repository will be deleted, so hence the copy here.

NOTE: This copy is being used for testing thepudds/fzgen and should not be relied upon more broadly.