It is easier to maintain code if macros use the fully qualified path to
types and functions instead of relying on code that uses the macro to
import said type or function.
The `ToHex` trait was replaced by either simple `Display`/`LowerHex`
where appropriate or `DisplayHex` from `bitcoin_internals` which is
faster.
This change replaces the usages and removes the trait.
Tobin C. Harding
Martin Habovstiak
Calvin Kim