D-like ranges for C++17 in a 2.5 klines single header file

[Per Nordlöw]

Here's a more compact encoding of D/C++20-like ranges packed into 
a 2.5 klines single header file:

The file `ranges.hpp` syntaxcheck in 0.3s with Clang 10 on Ubuntu 
20.04 on my 6 year old laptop.

https://github.com/simonask/rx-ranges/