Make the keys smaller

[irevoire]

Make the keys as small as possible by:

• Getting rid of the padding and the INTEGER_KEY thingy
• Making the index only 7-bit and shoving the mode as the last bit of the index
• Having only two modes instead of three, now the metadata is the last node of the tree nodes

---

Performances results:

baseline / main branch:

Starts to insert document in the database ...
Took 181.275834ms to insert the vectors
Took 217.843506416s to build
Took 218.02478225s in total to insert and build

Louis's query took 3.873666ms

smol_keys:

Starts to insert document in the database ...
Took 116.265167ms to insert the vectors
Took 201.482862333s to build
Took 201.5991275s in total to insert and build

Louis's query took 6.476875ms

And even though the query time is pretty small, the 3.8ms vs 6.4ms seems pretty consistent

[Kerollmops]

Please update the README to specify that users can use a u7 for their index, please?

And I am very worried about the x2 on the query time. Can you measure that just to understand the reason why? We can't double the query time to gain 10% of indexing time...

[Kerollmops]

Can we use a function for that, instead?

[Kerollmops]

Suggested change

	_ => unreachable!("Impossiburuu"),
	_ => unreachable!(),

[Kerollmops]

Can we use a non_meta function here, please?

[Kerollmops]

Can you add an infromation on what's the limit of this u8, please? Just a single doc line, we will refine that later.

[Kerollmops]

Suggested change

	let item_bytes = self.item.to_be_bytes();
	debug_assert_eq!(item_bytes.len(), output.len() - 1);
	output[1..].copy_from_slice(&item_bytes);
	let item_bytes = self.item.to_be_bytes();
	output[1..].copy_from_slice(&item_bytes);

This function will panic if the two slices have different lengths.
https://doc.rust-lang.org/stable/std/primitive.slice.html#method.copy_from_slice