Refactor simple func_s and sub_s into libqb/

[mkilgore]

libqb.cpp contains the definitions for most of the func_ and sub_ C++ functions which are called from the QB64-PE code. While lots of these hook into many of the various internal parts of libqb.cpp, others are fairly stand alone and either have no real dependencies, or only depend on things like qbs and the GFS API (#146, #148). These are things like many of the math functions, stuff like sub_chdir(), some file support functions, etc.

It would be good to get the ball rolling and start pulling these out into new source files somewhere in libqb/ (maybe libqb/src/subfuncs or something), we should be able to reduce the size of libqb.cpp somewhat, get the libqb/ layout more defined, and make it easier to see what the harder parts of libqb.cpp might be.

This also includes cleaning up some pulled in via other files, such as func__loadimage() as noted in #141. Those probably shouldn't be pulled into the libqb/ folder but instead kept in source files under internal/c/parts/. They will likely need some additional build logic for conditional compiling.