C++17ify nano/ "lib, node, rpc & secure subfolders"

[LenWilliamson]

Hello,

as suggested in this post, I would like to C++17ify everything under the nano/ directory. I will start with lib, node, rpc & secure subfolders. As suggested by @wezrule I will:

1. Add structured bindings
2. Add [[nodiscard]] to functions which return a bool
3. Inline class static variables which are declared in source files

If there are any other C++17 features I should implement, please let me know.