chore: move bitfield library to lib director

I'd presonally prefer to use a git submodule here but It doesn't quite
seem like git submodules are possible for individual files. I'll have to
check with FlorenceOS every once and a while to ensure that there are no
lingering soundness issues with the library.

Thanks to @N00byEdge for this wonderful library!

src/cpu.zig

@@ -1,6 +1,6 @@
 const std = @import("std");
 const util = @import("util.zig");
-const bitfield = @import("util/bitfield.zig");
+const bitfield = @import("bitfield");
 
 const Bus = @import("bus.zig").Bus;
 const Scheduler = @import("scheduler.zig").Scheduler;