Checksum for target account hash

[piotr-dziubecki]

Implement CEP-57.

There is already a draft PR #1557

Needs the following touch-ups before it is merged:

• Tests fixed since @afck and I changed the scheme to use our hash entropy efficiently
• Rebase to dev
• Optimizations
  • Replace blake2bhash which returns a Vec with something that returns [u8; 32] so we avoid using the heap
  • Reduce the number of passes over the hex string in decode to at most 2
  • Make it so decode doesn’t do any heap allocations
  • Simplify for loops to use iteration instead
• Remove all hex external crate dependencies
• Benchmarks: run cargo bench --bench transfer_bench -- --save-baseline 'dev' on dev branch, then on this branch run cargo bench --bench transfer_bench -- --baseline 'dev'