Remove the zero index from VarZeroVec

[Manishearth]

Fixes #5516

Also removes METADATA_WIDTH, we've set it to 0 and aren't using it. It's an unnecessary untested complication on our unsafe code.

[Manishearth]

I still need to run all the datagens and fix non-zerovec tests, but this works within zv.

[sffc]

Also removes METADATA_WIDTH, we've set it to 0 and aren't using it. It's an unnecessary untested complication on our unsafe code.

It was there to enable us to use FlexZeroVec as the index store. But, making VZV cheaper overall means that we can lean toward designs with more, shorter VZVs with Index8, instead of fewer, longer ones with Index16. We also now have ZeroTrie which covers some of these types of cases. I'm okay removing METADATA_WIDTH so long as we keep the door open to potentially re-adding a more compact index structure in the future.

[Manishearth]

It was there to enable us to use FlexZeroVec as the index store.

Yeah, that would require far more changes to the unsafe code of VZV than just METADATA_WIDTH, METADATA_WIDTH didn't particularly set the stage for anythig there code-wise.

It would be interesting encapsulating more and more of the format into the trait to allow for fancier custom formats.

[sffc]

To components.rs

[sffc]

Question: why did you expand the binary_search impl instead of using the std one?

[Manishearth]

The hacky impl was written assuming a slice to search on. Now I could still use a byte slice that I have cut out appropriately, but the indices slice that we currently use is no longer long enough for this to work, and it was going to get more complicated, so I decided to dispense with the weird hack. I can bring it back and make it work with the weird hack if desired.