switch to C++17

[MichaelRawson]

It's been about three years since we switched to C++14 in #197. C++17 has things we would like, so let's switch.

This:

1. Switches the compiler flags appropriately.
2. Fixes resulting warnings (std::result_of deprecated in favour of std::invoke_result)
3. Goes through the code fixing all "when we have C++17" comments - except for one in Hash.hpp @joe-hauns - I couldn't figure out what you meant, maybe you remember?
4. Replaces a few Lib::System functions that are now trivial in C++17.

I've asked anyone whose code I've touched to review. But anyone is welcome to review!

[joe-hauns]

Looks all good to me! Great to finally have c++17! Can't wait for structural binding declarations. That will make our iterator chains in the inference rules so much more readable!

Also I fixed the TODO you mentioned.

[MichaelRawson]

Thanks all for quick reviews! Merging.