src/: change c header identifiers (see #35)
src/io/*.{c,h}: added const qualifiers to unmodified pointers
src/io/sink_wavwrite.h: add do_multi, preset_samplerate, preset_channels, get_samplerate, and get_channels
src/io/sink*: add _close function, improve error messages
src/io/sink_wavwrite.c: add native basic wav writer