Add Index::merge_from + Index::insert exploits stuff for perf.

[Centril]

Description of Changes

This PR:

• Does some minor readability improvements and organization to indices.
• Adds Index::merge_from(&mut self, src: Self), which merges src into self. This will be used to speed up merging of transactions.
• Makes Index::insert to exploit a property for improved performance in itself and in Index::merge_from.
• Adds a trait invariant stating that no (key, ptr) occurs twice. This was always the case, but now it is a safety invariant and exploited for performance. This then is bubbled up to MutTxId and friends.

API and ABI breaking changes

None

Expected complexity level and risk

4 - This touches both Index::insert, Table, and MutTxId in terms of unsafe.

Testing

• Debug assertions are added for the unsafety in Index::insert.
• A new proptest merge_from_is_union is added that comprehensively exercises Index::{insert, merge_from, can_merge} together and for all index kinds and uniqueness.