[WIP] Changes to C++ unparser to facilitate SQL to C++

[EraYaN]

Tracking some changes here.

[mbdevpl]

Hey! Just FYI: I've been also working on the C++ unparser in the meantime - although changes are not in master yet. I'd definitely like to merge the efforts at some point ;)

[EraYaN]

Right my main blocker is the fact that an and gets output because it uses the basic unparser from horast for expressions. While not all compilers support this (MSVC for example). Although you can get it with a header include, and that works, it's not truly proper IMO. But I also couldn't bring myself to go and re implement all the unparsing logic for expressions.

The branch name is misleading at this time, since I only touched the C++ unparser. Also I saw that python 3 supports syntax with a * in front of a variable. So that could then be mapped to the same operator in C++.

But most of all, I'm just trying to make stuff work. (like char literals are bytes objects that happen to be one character.)

[EraYaN]

Closed in favour of #8