std.hash review: Update 1

[Johannes Pfau]

I implemented some of the suggestions, here's the list of changes:

Changelog:
* Add a new overload to the 'digest' function which accepts an
  InputRange
* Add a new convenience function 'hexDigest' which works just like
  'digest', but returns a string (also works with InputRanges)

  An open question is whether md5StringOf/md5HexOf aliases should be
  added (similar to md5Of)?
* Add a new convenience function 'startDigest' which returns an
  initialized digest
* New example for file hashing in idiomatic D
* Documented that Digests are always OutputRanges
* Added new examples using std.algorithm.copy & OutputRange interface
* Small optimization in toHexString & hexDigest: do not allocate if
  possible

TODO:
* move the package to std.digest (unless there are objections):
    std.hash.hash --> std.digest.digest
    std.hash.md   --> std.digest.md
    std.hash.sha  --> std.digest.sha
    std.hash.crc  --> std.digest.crc

* make sure the docs are consistent regarding names (digest vs. hash)


Code:
https://github.com/jpf91/phobos/tree/newHash/std/hash
https://github.com/jpf91/phobos/compare/master...newHash

Docs:
http://dl.dropbox.com/u/24218791/d/phobos/std_hash_hash.html
http://dl.dropbox.com/u/24218791/d/phobos/std_hash_md.html
http://dl.dropbox.com/u/24218791/d/phobos/std_hash_sha.html
http://dl.dropbox.com/u/24218791/d/phobos/std_hash_crc.html