Rename __builtin__ to __black_box__
builtin started out as a way to add intrinsics to the compiler. However, eventually I realised that it can also be used to write black-box interfaces. We should probably rename it to black box because of that, and possibly also consider adding a different mechanism for compiler intrinsics. Currently, intrinsic resolution is done very late, which means that error messages about generic items being builtin can only be reported during instanciation in hir_lowering.