Use `const_cast` to be able to pass it to sf_open_virtual() which has
a wrong parameter declaration.

Fixes memory leaks.

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

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.

Support these PCM formats natively, instead of letting libsndfile
convert everything to 32 bit.

Prepare for other sample formats.

Reduce header dependencies.