Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

The old struct config_param remains only for top-level string options.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.