function-keyword debug improvements

[Thorium]

@vzarytovskii pointed out that FSharp core team cannot use function keyword:

let myFun = 
   function 
   | Some x -> x 
   | None -> "nothing"

...but instead always write explicit match:

let myFun = fun itm -> 
   match itm with 
   | Some x -> x 
   | None -> "nothing"

...because easier debugging.

My suggestion would be following improvements:

• when you hover over "function"-keyword at debug-time, it should display the omitted parameter value as tooltip.
• you should be able to attach a debug-point to function keyword itself (if not working already).

[brianrourkeboll]

More generally, one could argue that it would be nice to have a debug point at every pattern-matching location, so that the pattern input could always be inspected even if it was not already bound to a name in scope.

I suspect that that might be a more invasive change to the compiler, though.