nix-vector-routing: Code and Performance optimizations

This MR has the patches for:

• Add unordered_map Ptr<NetDevice> to Ptr<IpInterface>
• Remove RunIf construct defined and use if constexpr instead (Works since C++17)
• Add a condition in subnet matching to check if remote address is link-local.
• Remove the C++ template specialization and use if constexpr instead. The reason for this is:
  • Template specialization was done to accommodate for some differences in IPv4 and IPv6.
  • Here, the RunIf was a viable option but we went with template specialization because of better code redability.
  • Due to C++17, we can use if constexpr which makes the code readability better than template specialization and remove any code redundancy.
  • Performance-wise, it should remain the same as both specialized functions and if constexpr will generate similar code (in previous case and this MR) for both the instantiations at compile time.

Signed-off-by: Ameya Deshpande ameyanrd@outlook.com