Changes from 2.15.2 to 2.16.0

• Use _fseeki64/_ftelli64/_stat64 on Windows for large file (>2 GB) support.
  Thanks to Abhi Jaiantilal (@ajaiantilal) for the report and help.
• Add 12-byte unshuffle for avx2. Thanks to Tom Birch (@froody).
• Add 12-byte sse2 unshuffle implementation. Thanks to Tom Birch (@froody).
• Better description of the Blosc2 format as a whole.