Proposal: Add isLeft/isRight to Data.Either

[Gabriel Gonzalez]

Although I provide "isLeft" and "isRight" in my "errors" package, count 
me against adding them to Data.Either.  I only include them in "errors" 
because it is supposed to be a "everything and the kitchen sink" utility 
library.

The API of standard libraries should provide guidance to new users for 
how to program idiomatically in Haskell and including those two 
functions will very likely lead new users astray from pattern matching, 
giving them a poor initial impression of Haskell.