SDK: Slot Hashes: Add queries using sol_get_sysvar

[buffalojoec]

Problem

Some Core BPF programs - such as Address Lookup Table - will need to be able to query
for a slot's corresponding hash or position in the SlotHashes sysvar.

Thanks to #1307, we can leverage the new sol_get_sysvar to enable this
functionality for BPF programs.

Summary of Changes

First add a general helper API for working with sol_get_sysvar. This can be used by
many sysvar APIs.

Next add a new API - SlotHashesSysvar - offering two queries: get and position.

Example Usage

fn process_instruction(/* ... */) -> ProgramResult {
    if let Some(hash) = SlotHashesSysvar::get(&0)? {
        msg!("Found Hash: {:?}", hash.to_bytes());
    }

    if let Some(position) = SlotHashesSysvar::position(&312)? {
        msg!("Found Hash Position: {}", position);
    }

    Ok(())
}

A Note on Use of sol_get_sysvar

It turns out it was actually cheaper on compute to load the entire buffer for SlotHashes
without deserializing, and implement binary search on the raw data. Working with chunks
at a time was more expensive on compute.

[buffalojoec]

@2501babe I based this off of your implementation for StakeHistory, but I was able to
cut some corners by avoiding bincode, since Hash is just bytes.

I think we can also maybe share the first commit if you are interested, since our methods
both need to use the same call.

[buffalojoec]

@joncinque @2501babe Requesting initial review as a draft before adding tests.

[joncinque]

Thanks for getting this up! It's good to see its potential usage

[joncinque]

This is starting to look very nice, just a few more bits to refine it

[joncinque]

Just two last nits, then this is good to go! I'm a big fan of how it's turned out

[joncinque]

[2501babe]

late to the party while i close out the new stake instructions but i love this, im going to rearrange my stake history stuff to also use your get_sysvar function