Improvemens to simpPRU

Building upon the work done in GSoC 2020 to make programming the PRU easier for beginners, this project aims to add tests, features, and more compatibility for simpPRU, the simple Python-like language that compiles to PRU C and runs natively on the PRU. More specifically, I am planning to add support for the BeagleBone Blue, add unit tests for every language feature, add hexadecimal support for ints, add support for modulo and bitshift operators, support for a new char/uint8 data type, support for a simple array structure, adding a more flexible for loop, and adding the ability to send more complex data in the form of strings through rpmsg and receive them in simppru-console. I will also improve the error handling in the parser and fix known issues in simpPRU. This will make the overall language more robust and complete, which will help beginners learning to use the PRU or experienced users prototyping something on the PRU.