Add structure-based request-sending API

[psychon]

Idea originally by @eduardosm. See #211 (comment).

Basically:

https://github.com/eduardosm/rust-xcbalt/blob/974b76a5e2e776f87a06fa0abc9fd0d3962696b3/examples/xi2_events.rs#L10-L36

Why would one want something like this?

According to clippy, some functions already have too many arguments: https://github.com/psychon/x11rb/blob/1e6289fb7ef20041038b8fca440520bd731aedac/code_generator_helpers/module.py#L181

Let's also look at a random create_window call:

x11rb/examples/simple_window.rs

Line 34 in 1e6289f

conn.create_window(screen.root_depth, win_id, screen.root, 0, 0, width, height, 0, WindowClass::InputOutput, 0, &win_aux).unwrap();

No idea what all of the 0 arguments are. Uhm.. there is border_width. I think that comes after width and height. Besides that, no idea. After having taken a look: Oh, I could have thought of x/y, but didn't. The last 0 is visual and 0 just happens to be the value for COPY_FROM_PARENT.

Anyway, the point is: This stuff is currently hard to understand.

The idea with this issue is not to replace the already existing API, but add a new alternative to it (perhaps behind some Cargo feature).

[psychon]

Implemented by #499