SNIP-52: Add `env.transaction.hash`

[assafmo]

Refs:

• https://github.com/SolarRepublic/SNIPs/blob/feat/snip-52/SNIP-52.md#notification-data-algorithms
• https://github.com/SolarRepublic/snip52-reference-impl
• https://demo.starshell.net

Cc @darwinzer0 @blake-regalia

Also fixed the test contracts to work with the new Rust memory optimizations.