Remove/inline case expressions when there is only one clause

[Abhiroop]

Here is an example program:

double : Int -> Int
double n = n * 2

main = double 5

This currently gets desugared to:

let v0 = \ v2 -> case v2 of {
  v1 -> v1 * 2
}
in v0 5

This can be further simplified to:

let v0 = \ v2 -> v2 * 2
in v0 5

[Rewbert]

When functions are desugared pattern matches are turned into case expressions, and right now I believe it assumes there to be more than one pattern. Should be fairly simple (famous last words) to make it recognise that if there is only one pattern and that pattern is a variable, no case is needed.