HACK: Tweak schema_updates to allow adding/removing non-unique indices

[Centril]

Description of Changes

You can now add and remove non-generated indices.
That is, the indices that you actually define with #[spacetimedb(index(...))].

API and ABI breaking changes

None

Expected complexity level and risk

3, some questions re. transactionality...

Testing

• Created a project with a table without an index, then added the index, republished, then removed the index and republished (HACK: Tweak schema_updates to allow adding/removing non-unique indices #1434 (comment)).

Review notes

• Hide whitespace changes on github.

[Centril]

I confirmed that this works with the following module I called foo:

use spacetimedb::{println, spacetimedb};

#[spacetimedb(table)]
pub struct Table {
    field: u64,
}

#[spacetimedb(init)]
pub fn init() {
    for i in 0..1_000 {
        Table::insert(Table { field: i });
    }
}

#[spacetimedb(reducer)]
pub fn contains(x: u64) {
    if Table::filter_by_field(&x).next().is_some() {
        println!("Found {x}");
    } else {
        println!("Didn't find {x}");
    }
}

While running spacetime start --in-memory, I did:

1. spacetime publish foo
2. spacetime call foo contains 500 ==> WARN iter_by_col_range without index: table Table has 1000 rows;
3. added #[spacetimedb(index(btree, name = "field", field))]
4. spacetime call foo contains 500 ==> no warning
5. removed #[spacetimedb(index(btree, name = "field", field))]
6. spacetime call foo contains 500 ==> WARN iter_by_col_range without index: table Table has 1000 rows;

[kim]

Seems to do the right things.

Maybe the smoke tests could be extended to cover what is accepted as an update and what is not (I don't think we can test that an index is present or absent).

Note that adding or dropping an index is not transactional. However, the modifications of the system tables are, so re-publishing a failed update should converge to the correct state.

[Centril]

Maybe the smoke tests could be extended to cover what is accepted as an update and what is not (I don't think we can test that an index is present or absent).

Filed #1438 for the smoketest.