Replace deprecated stdlib calls and establish minimum Rust version

[56quarters]

There are a number of warnings emitted now (1.33.0-nightly) about use of things in the standard library that are deprecated.

Instead of just replacing them, establish the minimum version of Rust required to use Cadence and document it. Perhaps establish a policy of supporting $current - 2 releases or something. Only once the minimum version is documented can we move on to replacing the deprecated methods.

[56quarters]

Results of running cargo test, cargo test -- --ignored, and cargo bench with various versions of Rust (2015 edition) with da9f98e of Cadence.

Commands

rustup run $ver cargo test
rustup run $ver cargo test -- --ignored

Results

• 1.24.0 - fail, u128 used by crossbeam unstable
• 1.25.0 - fail, u128 used by crossbeam unstable
• 1.26.0 - pass
• 1.27.0 - pass
• 1.28.0 - pass
• 1.29.0 - pass
• 1.30.0 - pass
• 1.31.0 - pass
• 1.32.0 - pass

[56quarters]

Closing this for now. The replacement for the deprecated calls that prompted this ticket was not introduced until 1.30 and so we can't replace them until we drop support for 1.29 which will be a little while.