fantasy-land: document use of S.lift2 and S.lift3 as combinators

[davidchambers]

sanctuary-js/sanctuary-site#92

In addition to documenting the use of lift2 and lift3 as combinators, this pull request standardizes the steps for deriving signatures of combinators from more general signatures:

1. Provide general signature:

   Functor f => (a -> b) -> f a -> f b

2. Replace type variable with Function x:

   (a -> b) -> Function x a -> Function x b

3. Convert to -> form:

   (a -> b) -> (x -> a) -> (x -> b)

4. Remove unnecessary parentheses:

   (a -> b) -> (x -> a) -> x -> b

5. Rename type variables:

   (b -> c) -> (a -> b) -> a -> c

[Bradcomp]

[Avaq]

Any reason to hold off merging this @davidchambers ?

[davidchambers]

Any reason to hold off merging this @davidchambers ?

No reason whatsoever. :)