I wanted to use this sysvar to get a recent block hash, but I didn't
know whether the first or the last entry contains the most recent block
hash.
By calling it for mainnet, printing the results, and comparing that to
the recent blocks on solanabeach.io/blocks, I discovered that the
entries are ordered from most recent to least recent. Document this to
save future readers the trouble.
* Upgrade Rust to 1.52.0
update nightly_version to newly pushed docker image
fix clippy lint errors
1.52 comes with grcov 0.8.0, include this version to script
* upgrade to Rust 1.52.1
* disabling Serum from downstream projects until it is upgraded to Rust 1.52.1
