Move Printable conformance into the type body.
Use .first/.rest instead of first()/dropFirst().
Remove first()/dropFirst().
Move ReducibleType into conformance into the type body.
Section for concatenation.
Move ReducerOf’s concatenation operator declaration to the bottom.
Document .first.
Document .rest.
Document uncons().
Move cases down to the bottom.
More thorough docs of the Cons case.
Document not using Stream.Nil directly.
Merge pull request #48 from robrix/tidy-stream

Tidy `Stream`