Append simple multithreading support

[slavaza]

This small improvement will allow the use of multitasking programming techniques on the smallest Arduino models. It implements iterative multitasking with execution time quantization.

[per1234]

From the comment on the previous PR (arduino/Arduino#7459 (comment)). It sounds like there is possible interest in your commit to add the yield() call to HardwareSerial::write() (bf53c99) but not in your other commit that adds a Yield library to the core.

By bundling the two only loosely related commits together in a single pull request you make it less likely that the first will be merged. It seems like the best approach for the second is to simply publish the library in a repository of your own and then Arduino users can install it if they like. You can make it even easier to install and more visible by requesting its addition to the Arduino Library Manager index.

[slavaza]

@per1234 You have the right to do what you want as developers. I just offered my solution, wanting to play with Arduino. Yes I will try to publish this library separately. I do not search popularity. I just want improve your project that I liked.

[CLAassistant]

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