This commit introduces `WrapDebug` in `internals` and updates `Witness`
debug implementation to use it. The previous `DebugElements` struct has
been removed in favor of an ad-hoc closure inside `WrapDebug`, which
formats witness elements as a debug list of hex-encoded values.
By abstracting out the "debug-print hex fields" pattern, we reduce
code duplication and improve maintainability.
Erick Cestari