news: release 0.8.0

• CompressionParameters now has a estimated_compression_context_size() method.
  zstd.estimate_compression_context_size() is now deprecated and slated for
  removal.
• Implemented a lot of fuzzing tests.
• CompressionParameters instances now perform extra validation by calling
  ZSTD_checkCParams() at construction time.
• multi_compress_to_buffer() API for compressing multiple inputs as a
  single operation, as efficiently as possible.
• ZSTD_CStream instances are now used across multiple operations on
  ZstdCompressor instances, resulting in much better performance for
  APIs that do streaming.
• ZSTD_DStream instances are now used across multiple operations on
  ZstdDecompressor instances, resulting in much better performance for
  APIs that do streaming.
• train_dictionary() now releases the GIL.
• Support for training dictionaries using the COVER algorithm.
• multi_decompress_to_buffer() API for decompressing multiple frames as a
  single operation, as efficiently as possible.
• Support for multi-threaded compression.
• Disable deprecation warnings when compiling CFFI module.
• Fixed memory leak in train_dictionary().
• Removed DictParameters type.
• train_dictionary() now accepts keyword arguments instead of a
  DictParameters instance to control dictionary generation.