Make `TxnHash` struct

[Isaac-DeFrain]

We should use pub struct TxnHash(pub String) over String

[trevorbernard]

We should strongly type all the things

[nathan-tranquilla]

There are some performance considerations around this. Currently we're passing around &str and refactoring to TxnHash(pub String) would result in many more memory allocations on the heap. There probably are some ways around this using Cow, but I think it is a little beyond me at this moment.

[trevorbernard]

Cow isn't necessary if you aren't mutating a shared resource. Just pass in the &TxnHash. This feels like prematurely optimization. Even better would be to use the txh hashes binary representation as the value rather than it's string-ified representation.