combine std.build and std.build.Builder into std.Build

I've been wanting to do this for along time.

lib/std/Build/RemoveDirStep.zig

@@ -0,0 +1,29 @@
+const std = @import("../std.zig");
+const log = std.log;
+const fs = std.fs;
+const Step = std.Build.Step;
+const RemoveDirStep = @This();
+
+pub const base_id = .remove_dir;
+
+step: Step,
+builder: *std.Build,
+dir_path: []const u8,
+
+pub fn init(builder: *std.Build, dir_path: []const u8) RemoveDirStep {
+    return RemoveDirStep{
+        .builder = builder,
+        .step = Step.init(.remove_dir, builder.fmt("RemoveDir {s}", .{dir_path}), builder.allocator, make),
+        .dir_path = builder.dupePath(dir_path),
+    };
+}
+
+fn make(step: *Step) !void {
+    const self = @fieldParentPtr(RemoveDirStep, "step", step);
+
+    const full_path = self.builder.pathFromRoot(self.dir_path);
+    fs.cwd().deleteTree(full_path) catch |err| {
+        log.err("Unable to remove {s}: {s}", .{ full_path, @errorName(err) });
+        return err;
+    };
+}