Remove CharIO in favor of `FormatWriter` and `write!` in `core`

[kvark]

This has been discussed a little, and we should investigate more for sure. I just don't know why CharIO is needed exactly. For my purposes of debug output via UART, the write! macro with FormatWriter implementation works perfect.

[farcaller]

There was no fmt in core (and libcore) at the time that was created. I'm totally fine with deprecating it (write! macro is just slightly more verbose).

[archaelus]

👍 I just implemented this on my teensy/k20 branch of zinc - https://github.com/archaelus/zinc/blob/k20_72/src/hal/k20/uart.rs#L188-L198 It's a tiny amount of code, and quite a bit easier to use than CharIO.