py/stream: Reuse write implementation for readinto.

This commit refactors some common code in the core stream implementation,
to reduce code size while retaining the same functionality.

With the factoring, `readinto`/`readinto1` could now support an additional
4th argument (like write) but it's best not to introduce even more CPython
incompatibility, so they are left as having a maximum of 3 args.

Signed-off-by: Damien George <damien@micropython.org>

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