1. Turn off MICROPY_CPYTHON_COMPAT, which includes a number of minor CPython compatibility features,
most of which have workarounds, but uses up significant flash.
2. Turn on MICROPY_PY_SYS_MAXSIZE, which implements sys.maxsize.
3. Turn on MICROPY_CAN_OVERRIDE_BUILTINS, which implements "_" as the most recent value in the REPL,
and also enables redefining builtins.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
