Support C++ concepts

This is more of a note for myself.

**Is your feature request related to a problem? Please describe.**
As of now, the compile-time error messages is very ugly for `app().registerHandler()`. As it depends of multiple layers of SFINAE to detect if the function have the correct signature. Which is annoying while developing applications.

**Describe the solution you'd like**
```c++
template
concept HTTPFunction = FunctionTraits::isHTTPFunction;
```

And put it somewhere before SFINAE.

**Additional context**
GCC doesn't label itself as supporting C++20 before GCC 11. And unlike coroutines, there's no way to detect if a compiler supports concepts. I'll wait till GCC 11's release to implement this.

(My clang doesn't support coroutines, so can't test weather it deal with coroutines correctly. It shouldn't be an issue, but I rather be on the safe side)