atomic_cell_type has various static make_*() methods which create a
serialized cell based on the parameters. This patch adds write_()
methods which mirror the existing make_*() ones, with the exception that
the write methods write into caller-provided buffer. The make methods
are refactored to call the appropriate write overload.
*_serialized_size() methods are added as well, to calculate how many
bytes the serialized data will take after the appropriate write call.
This allows code to write cells directly into a pre-arranged buffer,
perhaps even multiple ones into the same one.
Since the intended use-case this patch prepares for is serializing an
entire collection directly into a single buffer, only make variants
which are legal in collections are handled. I.e. counters are not.
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