This uses MP_REGISTER_ROOT_POINTER() to register the readline_history root
pointer array used by shared/readline.c and removes the registration from
all mpconfigport.h files.
This also required adding a new MICROPY_READLINE_HISTORY_SIZE config option
since not all ports used the same sized array.
Signed-off-by: David Lechner <david@pybricks.com>
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.