Refactor `list` into a separate source file in `libqb/`

[mkilgore]

list is a structure defined in libqb.cpp. It is fairly self-contained but used by a lot of the code. I don't personally think it's worth using in a lot of cases compared to various C++ data structures we have access too, but lots of code needs to interface with existing lists that won't be replaced any time soon, so making this properly available in libqb/ would help that.