Add Read cancellation

[kroppt]

Fixes #105 Related to #117

In this solution, I gave serial.Serial a new function called ReadContext, which is identical to Read, except it also takes in a context.Context as its first parameter.

I understand that there may be some concern over the API, given that Go developers as a whole have not decided on how to resolve the problem of cancellation for abstract file systems, but it seems OK to add this feature, because the interface is defined in this library and is not a generalized interface that is a part of the standard library. I am happy to talk through any concerns.

I tested this on both Linux and Windows. It probably also works for other Unix platforms, since their implementation is the same as Linux.

I cleaned up the Linux tests, since they weren't very careful. Additionally, I added Windows tests, which were a bit trickier. I didn't see an obvious way to create a dummy serial port, so the tests will run using the first serial port on the system, otherwise skipping the tests.

[soypat]

I'll note that this could best be solved by an external library that implements cancellation for any kind of io.Reader implementation. The way I've gone about dealing with time-limited readers is by using this API: https://github.com/soypat/cereal/blob/main/nonblocking.go

[kroppt]

I'll note that this could best be solved by an external library that implements cancellation for any kind of io.Reader implementation. The way I've gone about dealing with time-limited readers is by using this API: https://github.com/soypat/cereal/blob/main/nonblocking.go

It looks like it creates a long-running goroutine that handles reading and then buffers it for the primary code to read. I think that's a good approach. If I recall, I might have done something like this in the code I was writing. If you use channels to pass along those bytes, it seamlessly integrates with select semantics from the cancellation channel (which avoids having to poll). And if you need it buffered potentially infinitely, you can insert an intermediate goroutine that just buffers the incoming bytes. Boy, this is getting complicated! But I remember it worked well for me.

[KiddoV]

I am interested in this feature too, as sometimes I need to stop the reading manually instead of waiting for an error or the received byte = 0.