Add Const functor and const function

`const` function inspired by Runar's
[A Proper Constant Function in Scala](http://apocalisp.wordpress.com/2010/04/21/a-proper-constant-function-in-scala/)
blog post.
Const.const s/Id/Function0, make ConstFunctions sealed
Merge pull request #657 from adelbertc/const

Add Const functor and const function