Naming and design tweaks

[de-vri-es]

Global overview:

• Although I liked the idea of not exposing too much time details in the trait, having a stateful timeout requires the Transport implementation to always store the deadline. This means that you basically always need a wrapper type to implement the Transport trait.

  So instead I made the timeout non-stateful so we can implement the trait directly for serial2::SerialPort.

  This also opened the way for renaming the trait to SerialPort again.

• I didn't want the SerialPort impls to return dynamixel2 errors, but only their own error. So I removed the Timeout error variant and instead added a static is_timeout_error() function to the SerialPort trait. Similarly I removed the InitializeError and just have the functions return SerialPort::Error.

So far, these are the high level changes. I also want to rename Bus, Device and Messenger, but I'm out of time for today :p

@omelia-iliffe: Could you look over the changes and see what you think?

[omelia-iliffe]

Awesome. This all looks great. I see what you meant about having an Instant type now. I was worried I wouldn't be able to make this work with embedded platforms but I found solutions for both rp2040-hal and stm32g4xx_hal I am targeting.
You can see the updated rp2040 example here: https://github.com/omelia-iliffe/dynamixel2-rs/tree/no_std_example/examples/rp2040. It compiles and I will test it when I get a chance.