Optimize W256, Addr conversion to ByteString

[elopez]

Description

The previous code caused increased memory allocations and unnecessary garbage collection.

Echidna built with this change showed a ~12% improvement in execution time on 1000000 runs in a test codebase.

Checklist

• [ ] tested locally
• [ ] added automated tests
• [ ] updated the docs
• [ ] updated the changelog

[arcz]

That's cool!