Flatten and simplify the picosat C lib (make go-gettable)

I started looking at pigosat, and thought the first thing I would do is try to make it go-gettable. This uses the same approach as pygosat, where it only keeps the necessary C source right in the root, and I removed the subdir. 
cgo will only pick up C/C++ automatically if it is right next to the source. In my other projects that use cgo to do C/C++ bindings, I ended up following Gustavo Niemeyer's approach used in [go-qml](https://github.com/go-qml/qml/blob/master/all.cpp). But since there are only a few source files, it was easier to just drop them into the root.