Refactor zstd Decoder

[klauspost]

Currently working on refactoring the zstandard decoder. Goals of the refactor:

• Have fully singlethreaded decoding as option zstd: add no-goroutine option to encoding #264
• Simplify data flow.
• Split multithreaded decoding into: (continuing zstd split sequence decode/execute #100)
  • block decoder/splitter
  • literal decoding
  • sequence decoding
  • sequence execution
• Allow multiple frames to be decoded concurrently.

[MarkusFreitag]

Currently the *zstd.Decoder has a Close method without a return value and so does not fulfill the io.ReadCloser interface. Is this something that can be changed during the refactoring?

I'm not sure whether this belongs to this issue, if not I'm happy to create a new one

[klauspost]

@MarkusFreitag No. That would be a breaking change. Use IOReadCloser.

It shouldn't be used like a standard ReadCloser, so I on purpose does not support the interface.