Add Wrapper types for txid/block hash

[TheBlueMatt]

People are always confused that the byte order is swapped when logging/printing to hex, so we should have a wrapper type that does that.

[tnull]

Just as an idea: would it make sense to add such types in the regular lightning or a lightning-types crate to have consistency and DRYness over multiple projects?

[TheBlueMatt]

We could, but we currently use the rust-bitcoin types which makes sense, so I'm not sure if we should be rebuilding a wrapper that serves the same purpose on the lightning side.