On the log-addressing branch:  Implement the packed_stream_t object
used to read 7b/8b encoded data as used in the format 7 index.
This is committed separate from its usage to make review easier.

* subversion/libsvn_fs_fs/index.c
  (): new file
  (MAX_NUMBER_PREFETCH): new constant
  (value_position_pair_t,
   packed_number_stream_t): packed stream data structures
  (stream_error_create,
   packed_stream_read,
   packed_stream_open,
   packed_stream_close,
   packed_stream_get,
   packed_stream_seek,
   packed_stream_offset): implement packed stream logic
  (encode_uint,
   encode_int,
   decode_int): data conversion utilities

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1511365 13f79535-47bb-0310-9956-ffa450edef68