Prefer vertical decomposition

[sfogarty]

https://github.com/TU-CSCI2322-FL22/solver-the-connectors/blob/ec3768dbb3f7b8de3314ef33076437f4b7b7cd66/ConnectFour.hs#L54-L63

You don't need the accumulator list.

 availableMoves :: Board -> [Move] 
 availableMoves brd =  
     aux brd 1 
     where 
         aux :: Board -> [Move] -> Int -> [Move] 
         aux (Board [] clr) cnt = []
         aux (Board (c:cs) clr) cnt =   
             if length c < rows  
             then cnt:(aux (Board cs clr) (cnt+1))
             else aux (Board cs clr) (cnt+1) 

Also, I encourage "vertical" decomposition, i.e.

 availableMoves :: Board -> [Move] 
 availableMoves (Board cols clr) =  
     [num | (col, num) <- labelCols cols 1, length col < rows]
     where 
         labeledCols [] num = []
         labeledCols (c:cs) num = (c,n):(labeledCols cs (num+1)