Support struct tuples

[jindraivanek]

Struct tuples is not supported (System.Exception: Unexpected expression: Microsoft.FSharp.Compiler.Ast+SynExpr+StructTuple).

Small snippet of what we need to support:

let g : struct (int*int) = struct (1,1)
let z = fun (struct (u, v): (struct (int*int))) -> 5
let t = struct (1,2)

match t with
| struct (x,y) -> ()