expands number of erasure coding shreds in the last batch in slots (#16484)

Number of parity coding shreds is always less than the number of data
shreds in FEC blocks:
https://github.com/solana-labs/solana/blob/6907a2366/ledger/src/shred.rs#L719

Data shreds are batched in chunks of 32 shreds each:
https://github.com/solana-labs/solana/blob/6907a2366/ledger/src/shred.rs#L714

However the very last batch of data shreds in a slot can be small, in
which case the loss rate can be exacerbated.

This commit expands the number of coding shreds in the last FEC block in
slots to: 64 - number of data shreds; so that FEC blocks are always 64
data and parity coding shreds each.

As a consequence of this, the last FEC block has more parity coding
shreds than data shreds. So for some shred indices we will have a coding
shred but no data shreds. This should not cause any kind of overlapping
FEC blocks as in:
https://github.com/solana-labs/solana/pull/10095
since this is done only for the very last batch in a slot, and the next
slot will reset the shred index.

core/src/window_service.rs

@@ -636,8 +636,7 @@ mod test {
         parent: Slot,
         keypair: &Arc<Keypair>,
     ) -> Vec<Shred> {
-        let shredder = Shredder::new(slot, parent, 0.0, keypair.clone(), 0, 0)
-            .expect("Failed to create entry shredder");
+        let shredder = Shredder::new(slot, parent, keypair.clone(), 0, 0).unwrap();
         shredder.entries_to_shreds(&entries, true, 0).0
     }