Improve the pack file API, support conduit 0.8.4, support for short hashes, add mirage sync support

• Compatibility with cohttp.0.18. (#80 by @rgrinberg)
• Simplify the mirage sync API to use conduit 0.8.4 (breaking API changes)
• Change ogit cat-file to behave exactly as git cat-file
  The previous command is renamed to ogit cat (#75 by @codinuum)
• ogit now supports short hashes instead of full SHA1 (#75 by @codinuum)
• Add Git.Pack.Raw.read to read raw pack files (#75 by @codinuum)
• Git.Pack_index.t now uses a cache of entries. This is more efficient
  than the previous representation (#75 by @codinuum)
• Add Git.Pack_index.mem to find an entry in the pack index cache
  (#75 by @codinuum)
• Add Git.Pack_index.find_offset to find an offset in the pack index
  cache (#75 by @codinuum)
• Add Git.Packed_value.to_value to unpack a value stored in a pack file
  (#75 by @codinuum)
• Support synchronisation for MirageOS unikernels (#70)