Generating multiple build types in one project

[dcbaker]

I was thinking about this because Visual Studio supports this natively. It's a really hard problem for the reference implementation because of it's implementation details, but I think we could actually solve it fairly easily:

1. hold off on lowering away buildtype, b_vscrt, b_debug, b_optimisation (and possibly a few others) as long as possible
2. clone the IR into 4 separate IRs, one for each buildtype
3. rename all of the targets into foo_{buildtype}
4. set the default all in ninja to be all_{buildtype}, but the user can run ninja -C builddir all_{different_build_type} to get that instead

profit?