Add documentation aliases for filehash-RDS2 class
Add compression and xdr slots for filehashRDS2 class
Merge branch 'rds2' of github.com:rdpeng/filehash into rds2

* 'rds2' of github.com:rdpeng/filehash:
  Export RDS2 class
  Treat 'status' as possible vector
  Formatting
  gsub with useBytes = FALSE is very slow
  Check that object names can be unmangled
cache 1. object names, 2. path on disk and 3. length in an environment

* rather than caching object name/path as separate objects in an environment
* now names(x) is essentially instantaneous and length is still almost as fast
regression test for rds2
Assume directory layout with 1 layer of subdirs like git objects directory

* speedup on creation
Simplify (and speed up) handling of the objlist

* no need to get and assign the whole object from the environment
* no need to store list as separate object
* a list is still somewhat slower than it might as way of storing 
  key/file location pairs but in my tests <10% of the time is spent on this.
Merge remote-tracking branch 'jefferis/rds2' into rds2

* jefferis/rds2:
  Simplify (and speed up) handling of the objlist
  Assume directory layout with 1 layer of subdirs like git objects directory
  regression test for rds2
  cache 1. object names, 2. path on disk and 3. length in an environment
  Add compression and xdr slots for filehashRDS2 class