Change Hash subclasses to be Converters.

CHANGELOG.md

-# Changelog
+## 0.9.2
 
-## next
+* `Hash`, `MD5`, `SHA1`, and `SHA256` now implement `Converter`. They convert
+  between `List<int>`s and the new `Digest` class, which represents a hash
+  digest. The `Converter` APIs—`Hash.convert()` and
+  `Hash.startChunkedConversion`—should be used in preference to the old APIs,
+  which are now deprecated.
 
-* Hashing (md5, sha1, sha256) now works correctly for input sizes
-  up to 64 bits.
-* Small example added to generate message digests for files.
+* Top-level `sha1`, `sha256`, and `md5` fields have been added to make it easier
+  to convert

[Bob Nystrom, 2015/09/22 17:47:31]

" to convert..."?

[nweiz, 2015/09/22 21:37:33]

Done.

+
+* Hashing now works correctly for input sizes up to 2^64 bytes.
+
+### Deprecations
+
+* `Hash.add`, `Hash.close`, and `Hash.newInstance` are deprecated.
+  `Hash.convert` should be used for hashing single values, and
+  `Hash.startChunkedConversion` should be used for hashing streamed values.
 
 ## 0.9.1
 
 * Base64 convert returns an Uint8List
 * Base64 codec and encoder can now take an encodePaddingCharacter
 * Implement a Base64 codec similar to codecs in 'dart:convert'
 
 ## 0.9.0
 
 * ChangeLog starts here.