sys/termios: add cross-platform API for arbitrary baud rates

[tmplt]

trafficstars

This commit adds

pub struct ArbitraryBaudRate(pub u32);
impl TryFrom<ArbitraryBaudRate> for BaudRate { ... }

such that arbitrary baud rates can be specified on systems that otherwise do not support it (e.g. Linux) via abstraction.

The one problem I see with this implementation is the duplication of the BaudRate enum. Removing the duplication would require a new macro, which may be too complex for this feature to warrant implementation.

[tmplt]

Alan Somers @.***> writes:

Why add the ArbitraryBaudRate structure? I think implementing TryFrom<u32> directly on BaudRate should be fine. TryFrom is already implemented but the u32 is the C representation of the underlying B* enum. E.g. assert_eq!(BaudRate::try_from(14), Ok(Baudrate::B115200)), IIRC.