Reduce block code (#34)

Remove BlockRng64. Make BlockRng generic.
Add sealed trait le::Word.

Author: dhardy

CHANGELOG.md

@@ -16,7 +16,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Removed optional dependency `serde` ([#28])
 - Add `SeedableRng::fork` methods ([#17])
 - Rename trait `block::BlockRngCore` to `block::Generator` and associated type `Results` to `Output`; remove assoc. `type Item` and remove type bounds ([#26])
-- Add `fn drop` to trait `block::Generator` (#35)
+- Add `fn drop` to trait `block::Generator` ([#35])
+- Remove `BlockRng64` ([#34])
+- Remove impl of `RngCore` for `BlockRng`, making the latter more generic ([#34])
+- Add trait `le::Word` ([#34])
 
 ### Other
 - Changed repository from [rust-random/rand] to [rust-random/core].
@@ -31,6 +34,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#17]: https://github.com/rust-random/rand-core/pull/17
 [#26]: https://github.com/rust-random/rand-core/pull/28
 [#28]: https://github.com/rust-random/rand-core/pull/28
+[#34]: https://github.com/rust-random/rand-core/pull/34
 [#35]: https://github.com/rust-random/rand-core/pull/35
 
 [rust-random/rand]: https://github.com/rust-random/rand