1. Add a new Upsilon MicroPython standard library in the linux/
subdirectory. This puts all the submodules into classes with methods
for ease of access.
2. Totally rewrite mmio.py code generation. Instead of just dumping
registers, the build system now instantiates classes which
encapsulate the module in question.
3. Split the PicoRV32 special register interface away from the PicoRV32.
It is now the PeekPokeInterface, which will be used in the future to
implement register control for Waveform and SPI.
4. Integrate Waveform into the design. Has not been tested yet.
