`InOrderIndex::idx` is `usize`

We want to compile the client to both WASM environments and native environments. The blockchain MMR is designed to have `u32::MAX` leaves, which makes the max `idx` of `InOrderIndex` go over that limit.
In WASM, `usize` is 32-bit so we are inherently limited when we need to store/retrieve nodes from the database.
This is something to tackle upstream on the crypto crate, but AFAIK, this doesn't block us for the next ~2^31 blocks which is many years away. Still, adding this issue for visibility (and see if anyone has any comments).