py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for. A
port will also provide mphalport.h with further HAL declarations.
Reference MCU is dsPIC33J256GP506 with 256k ROM and 8k RAM, on the dsPIC
DSC Starter Kit board. The REPL works, GC works, pyb module has LED and
Switch objects. It passes some tests from the test suite (most it can't
run because it doesn't have the Python features enabled).
