py/gc: Add new MICROPY_GC_SPLIT_HEAP_AUTO "auto grow heap" mode.

When set, the split heap is automatically extended with new areas on
demand, and shrunk if a heap area becomes empty during a GC pass or soft
reset.

To save code size the size allocation for a new heap block (including
metadata) is estimated at 103% of the failed allocation, rather than
working from the more complex algorithm in gc_try_add_heap(). This appears
to work well except in the extreme limit case when almost all RAM is
exhausted (~last few hundred bytes). However in this case some allocation
is likely to fail soon anyhow.

Currently there is no API to manually add a block of a given size to the
heap, although that could easily be added if necessary.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

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