Implements dictionary union according to PEP 584's specifications, minus
the fact that dictionary entries are not guaranteed to be in insertion
order. This feature is enabled with MICROPY_CPYTHON_COMPAT.
Includes a new test.
With the assistance of Fangrui Qin <qinf@purdue.edu>
Signed-off-by: Rayane Chatrieux <rayane.chatrieux@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>
