Expose USB writer

[mciantyre]

The PR lets users split() the USB handle. When you split the handle, you get back a USB reader and USB writer. You can use the writer to send formatted / raw data back to the USB serial host.

See the new usb_writer.rs example for more information.

use core::fmt::Write;

let mut p = bsp::Peripherals::take().unwrap();
let (mut reader, mut writer) = p.usb.split();
writeln!(writer, "Hello world! 3 + 2 = {}", 3 + 2).unwrap();

[mciantyre]

bors r+

[bors[bot]]

Build succeeded:

• clippy
• format
• template
• precompiled