pod: Add in-place stake history deserialization

[joncinque]

Problem

The stake history sysvar is too large to deserialize on-chain.

Solution

Using pod types, create an in-place deserializer for StakeHistory. It's a bit goofy, but it should give an overall look at how it can be done. A few quirks about this, which is why it's only a draft:

• bincode serializes vector sizes as a u64, whereas most other tools do a u32, which is why there's a new type parameter on PodSlice and a new (goofy) conversion function into usize for the pod number types
• the stake program types might need to start taking in the Pod types for things like inflation calculation, I haven't looked that far into it

Let me know how this looks!

[joncinque]

Reopening in case it might still be useful

[buffalojoec]

I may be interested in this.

[2501babe]

@buffalojoec you may be interested in solana-labs/solana#35252 also

[buffalojoec]

@buffalojoec you may be interested in solana-labs/solana#35252 also

Ohhh yeah. I didn't realize you were tackling the sysvar, too!