feat: add into_inner for writer

[pepperoni505]

In my use case, I need to call into_inner on the WavWriter struct, which didn't exist.

Related to https://github.com/ruuda/hound/pull/4 but for the writer

[k0nserv]

@ruuda I'm also interested in this. It's particularly useful when writing to an in memory buffer e.g. Cursor<Vec<u8>>