WIP: Index insert

[jussisaurio]

works for trivial cases, but breaks on tree balancing. since at least @pereman2 is rewriting balancing this might be blocked until that is done?

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Main changes:

1. Add is_index to BTreeCursor
2. Remove assumptions of there always being a rowid, use BTreeKey enum instead
3. Add index insertion instructions and logic in insert.rs to insert to indexes if any exist
4. Make simulator create tables with PRIMARY KEY occasionally

Change no. 4 revealed that basic index insertion works, but breaks during balancing. i didn't look at the balancing logic too much due to it being rewritten (I think), will put more effort into it when balance rewriting is done

@pereman2 FYI