Remove table copies from tree_sequence_t and add options to share tables about tskit HOT 7 CLOSED

I think this would be very handy, especially for people working with large tables.

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Let's close this - I don't think we want to make any more changes to the semantics post 1.0.

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We should consider whether we want to do this for 1.0, and if it's something that we'd need to introduce backward incompatible changes later to support. I've added it to the 1.0 milestone for now.

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From an API perspective, what we need to enable this is a flag to say "transfer ownership of this table collection", where the tree sequence would tree the table collection at the end of it's lifetime. This would allow us to implement load more efficiently.

Because we need to support this behaviour, the tables parameter and struct member must remain non-const. So the change that we need to make is to drop the const qualifier on tsk_treeseq_init.

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I think this is done now @benjeffery?

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Not completly as the issue talks about borrowed references. We have only implemented stealing the reference. If we think that's enough then this could still be closed.

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I think a stolen reference is good enough. On the C side, managing memory safety of a tree sequence if a mutable table collection is "live" will be annoying. It may be a win to discourage such things.

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