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.

Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

Variable-length arrays are a C-only feature.

These are used by DSD-streaming servers.  For example, MiniDLNA uses
"audio/x-dsd".

The scan methods must be "noexcept".

Replaces decoder_data() and others.

Replaces decoder_initialized().

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.

Check for STOP before decoding the first chunk.  This reduces the
command latency.

Remember the chunk's total size.

Nobody cares.

Eliminate the loop from dsdiff_stream_decode().  It makes the code
complex, real-world files with multiple DSD chunks are outside of the
specification, and the "chunk_size" variable would be bogus anyway.

Reduce header dependencies.

They are only used inside dsdiff_read_metadata_extra().