Downsize `TxIdx` (`= BlockSize`) and `TxIncarnation`

[hai-rise]

• We can downsize TxIdx (= BlockSize) to u32.
• We can downsize TxIncarnation to u16 or even u8 and fall back to sequential execution on overflowing. Overflowing may even be acceptable.

While the code is easy, it is non-trivial to reason and show that this improves performance, as moving away from usize is not always free.