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!

build.zig

@@ -12,6 +12,10 @@ pub fn build(b: *std.build.Builder) void {
     const mode = b.standardReleaseOptions();
 
     const exe = b.addExecutable("zba", "src/main.zig");
+
+    // Bitfield type from FlorenceOS: https://github.com/FlorenceOS/
+    exe.addPackage(.{ .name = "bitfield", .path = .{ .path = "./lib/util/bitfield.zig" } });
+
     exe.setTarget(target);
     exe.setBuildMode(mode);
     exe.install();