Reduce allocations in crypto_rustcrypto.rs

[bjoernQ]

In crypto_rustcrypto.rs there were a few unnecessary .to_vec() calls on byte slices.

This was causing a lot of heap-memory fragmentation which became a problem on a very resource constrained device. Turned out these allocations are totally avoidable.

Side note: There were two failing tests before (and after) my changes:

• data_model::long_reads::test_long_read_subscription_success
• data_model::long_reads::test_long_read_success

I haven't had time to look into these but it's unlikely they relate to my changes.